Comorbid Autism, Anxiety, and ADHD in a Preadolescent Boy: Clinical Opportunities.

Background Attention-Deficit/Hyperactivity Disorder (ADHD) is the most common neurodevelopmental disorder in children (Polanczyk, Willcutt, Salum, Kieling, & Rohde, 2014) and is highly comorbid with Autism Spectrum Disorder (ASD) (Green et al., 2015; Kotte et al., 2013). Although it is well established that children with ADHD or ASD and their families experience poorer functioning including child and parent mental health problems, child peer problems, poorer family quality of life (FQoL) and parenting difficulties, it is unknown how comorbid ASD symptoms contribute to child and family functioning in children with ADHD. It is important to understand which comorbidities contribute to poorer child and family functioning to guide treatment planning. Aims This study aimed to examine the prevalence of ASD symptoms in children with ADHD and the association between ASD symptoms and child and family functioning across three connected studies. The specific aims of each study are outlined below. Study 1. To examine the prevalence and type of ASD symptoms (social interaction, communication and stereotyped behaviour) in children with ADHD and non-ADHD controls. Within the ADHD group only, we also examined the relationship between ADHD subtype, hyperactive/impulsive and inattentive symptoms, ADHD symptom severity and child gender and ASD symptom severity. Study 2. To examine the association between ASD symptoms and (a) social functioning; (b) mental health; (c) quality of life and (d) sleep, in children with and without ADHD. Study 3. To examine the association between ASD symptoms (measured dimensionally) in children with and without ADHD and a broad range of family functioning variables and to examine differences between ADHD+ASD, ADHD and control groups on family functioning variables. Methods Participants were 6-10 year old children (164 ADHD; 198 non-ADHD control) attending 43 schools in Melbourne, Australia, who were participating in the Children’s Attention Project. ADHD was assessed in two stages using the parent and teacher Conners’ 3 ADHD index and the Diagnostic Interview Schedule for Children IV (DISC-IV). ASD symptoms were identified using the Social Communication Questionnaire (SCQ). Child functioning measures were social functioning (Strengths and Difficulties Questionnaire (SDQ), mental health (DISC-IV, SDQ), quality of life (QoL: Pediatric Quality of Life Inventory 4.0) and sleep problem severity. Family functioning outcome variables were parent mental health, family quality of life (FQoL), and scales assessing couple conflict, couple support and parenting behaviours. Unadjusted and adjusted linear and logistic regression examined continuous and categorical outcomes, respectively. Results Study 1. Children with ADHD had more ASD symptoms than non-ADHD controls (adjusted mean difference = 4.0, 95% confidence interval (CI) 2.8; 5.3, p < 0.001, effect size = 0.7). Boys with ADHD had greater ASD symptom severity than girls with ADHD (adjusted mean difference = 2.9, 95% CI 0.8; 5.2, p = 0.01, effect size = 0.4). Greater ADHD symptom severity was associated with greater ASD symptom severity (regression co-efficient = 1.6, 95% CI 1.2; 2.0, p < 0.001). No differences were observed by ADHD subtype. Greater hyperactive/impulsive symptoms were associated with greater ASD symptoms (regression coefficient = 1.0; 95% CI 0.0; 2.0, p = 0.04) however, this finding attenuated in adjusted analyses, which accounted for parent educational attainment, socioeconomic status, child internalising and externalising comorbidities (p = 0.45). Study 2. Each standard deviation (SD) increase in SCQ scores was associated with a 6.7 unit reduction in QoL (p < 0.001) and greater parent and teacher-reported peer problems, emotional and conduct problems. For every SD increase in SCQ scores, internalising (OR = 1.8, 95% CI 1.3, 2.6, p = 0.001) and externalising disorders (OR = 1.5, 95% CI 1.1, 2.1, p = 0.02) increased, as did moderate/severe sleep problems (OR = 1.5, 95% CI 1.0, 2.2, p = 0.04). Most findings held in analyses adjusting for socio-demographic factors, ADHD symptom severity, and comorbidities (when not the outcome), with the exception of externalising disorders and sleep problems. Study 3. In unadjusted dimensional analyses, higher ASD symptoms were associated with more couple conflict (p = 0.04) and poorer FQoL for all subscales (p ≤ 0.001), with non-significant trends for less couple support (R2 = 0.10, p = 0.06), more hostile parenting (R2 = 0.02, p = 0.06) and poorer parent mental health (R2 = 0.02, p = 0.07). In adjusted dimensional analyses, higher ASD symptoms were only associated with poorer FQoL, across all subscales only (p ≤ 0.01). The trend association between ASD symptoms and parent mental health attenuated due to meaningful associations with comorbid internalising disorder (p = 0.003) and ADHD symptom severity (p = 0.05). The trend association between ASD symptoms and hostile parenting attenuated due to significant associations with comorbid externalising disorders (p = 0.002), lower parent education attainment (p = 0.03) and greater ADHD symptom severity (p = 0.04). Less couple support attenuated due to a significant association with socioeconomic status (p = 0.004). In unadjusted categorical analyses, parents of children with ADHD+ASD reported more couple conflict (p = 0.04), less couple support (p = 0.001), poorer FQoL (p <0.001) and a non-significant trend for greater mental health difficulties (p = 0.07), compared to the ADHD group. In adjusted categorical analyses, parents of children with ADHD+ASD had poorer parent self-efficacy (p = 0.02), poorer FQoL (p < 0.05) (p < 0.05) and a non-significant trend for less couple support (p = 0.06), compared to parents of children with ADHD. In unadjusted categorical analyses, family functioning was significantly poorer for the ADHD and ADHD+ASD groups, compared to controls for most outcomes (p <0.001). In adjusted categorical analyses, all findings attenuated except FQoL was significantly poorer for the ADHD and ADHD+ASD groups, compared to controls. Conclusion ASD symptoms are common, and associated with poorer functioning in children with ADHD. It is important for clinicians working with children with ADHD to identify and manage ASD symptoms, given that they exacerbate functional impairments in this already vulnerable group. The relationship between ASD symptoms and broader family functioning appears to be largely driven by internalising and externalising disorders, ADHD severity, and socioeconomic status. Poorer FQoL appears to be independently associated with ASD symptoms in children with ADHD.

Children with attention deficit hyperactivity disorder(ADHD) or autism spectrum disorder (ASD) individuallyand those with co-occurring ADHD and ASDexperience higher rates of total anxiety and psychiatric comorbidities such as gender dysphoria and locomotor skillscompared to their typically developing (TD) peers. In this study, it was hypothesized that youth with comorbid ADHD and ASD would experience higher levels of overall anxiety, specifically separation, generalized, and social anxiety. A literature review of relevant studies published from2007 to 2020 was performed, with a search involvingkey terms such as "Anxiety," "ADHD" and "ASD'". It was discovered that individuals with ADHD or ASD had higher levels of anxiety compared to their peers. Furthermore, children who have co-occurring ADHD and ASD had more serve levels of anxiety than children with an individual diagnosis of ADHD or ASD. Children with ASD, ADHD, and co-occurring ADHD and ASD had a higher prevalence of gender dysphoria and impaired locomotor skills, which lead to higher levels of psychiatric comorbidities seen in this population. It can be hypothesized psychiatric comorbidities could also have implications for the high anxiety levels seen in this population but further research is needed to confirm this.

Interoception refers to the awareness of internal physiological state. Several previous studies reported that people with autism spectrum disorders (ASD) and adults with attention-deficit/hyperactivity disorder (ADHD) have diverse patterns of interoception, but the extent of literature is limited and inconsistent. This study aimed to investigate the interoceptive accuracy (IA) in children with ASD, children with comorbid ASD and ADHD, and typically developing (TD) children with high and low levels of autistic traits. We administered the eye-tracking interoceptive accuracy task (EIAT) to 30 children with ASD, 20 children with comorbid ASD and ADHD, and 63 TD controls with high and low levels of autistic traits. Parent-report scales concerning ASD and ADHD symptoms were collected. ASD children with and without comorbid ADHD both exhibited lower IA than TD children. Reduced IA was also found in TD children with high-autistic traits relative to those with low-autistic traits. IA was negatively correlated with autistic and ADHD symptoms. Atypical cardiac interoception could be found in children with ASD. Difficulties in sensing and comprehending internal bodily signals in childhood may be related to both ASD and ADHD symptoms. LAY SUMMARY: The present study examined interoceptive accuracy (IA) in children with autism spectrum disorders (ASD), children with comorbid ASD and attention-deficit/hyperactivity disorder (ADHD), and typically developing (TD) children with high and low levels of autistic traits. ASD children with and without comorbid ADHD both exhibited lower IA than TD children. TD children with high-autistic traits exhibited decreased IA compared to those with low-autistic traits. These resultshave implications for understanding sensory atypicality found in ASD and ADHD.

BackgroundAttention deficit hyperactivity disorder (ADHD) is a common psychiatric disorder in children that can extend into adulthood and that is often associated with a variety of comorbid psychiatric disorders.AimAssess the comorbidity of ADHD with anxiety disorders and depressive disorders in school-aged children, and the relationship of the severity of ADHD, anxiety, and depressive symptoms in children who have ADHD with the severity of the corresponding symptoms in their parents.MethodsA two-stage screening process identified children 7-10 years of age with and without ADHD treated at the Xin Hua Hospital in Shanghai. ADHD and other DSM-IV diagnoses were determined by a senior clinician using the Schedule for Affective Disorder and Schizophrenia for School-Aged Children (K-SADS-PL). One parent for each enrolled child completed three self-report scales: the ADHD Adult Self Report Scale (ASRS), the State-Trait Anxiety Inventory (STAI), and the Beck Depression Inventory (BDI). In total 135 children with ADHD and 65 control group children without ADHD were enrolled; parents for 94 of the children with ADHD and 63 of the children without ADHD completed the parental assessment scales.ResultsAmong the 135 children with ADHD, 27% had a comorbid anxiety disorder, 18% had a comorbid depressive disorder, and another 15% had both comorbid anxiety and depressive disorders. Parents of children with ADHD self-reported more severe ADHD inattention symptoms than parents of children without ADHD and were more likely to meet criteria for adult ADHD. Mothers (but not fathers) of children with ADHD had significantly more severe trait anxiety and depressive symptoms than mothers of children without ADHD. Among children with ADHD, the severity of ADHD symptoms was not significantly correlated with the severity of ADHD symptoms in parents, but depressive symptoms and anxiety symptoms in the children were significantly correlated with the corresponding symptoms in the parents.ConclusionSchool-aged children with ADHD commonly suffer from comorbid anxiety and depressive disorders, and the severity of these symptoms parallels the level of anxiety and depressive symptoms in their parents. Self-reported symptoms of ADHD are significantly more common in parents of children with ADHD than in parents of children without ADHD. Longitudinal studies are needed to disentangle the genetic, biological, and social factors responsible for these complex inter-relationships.

Introduction Over recent decades, research has identified both overlapping and distinct characteristics, risk factors, and genetic as well as neurobiological correlates associated with Attention-Deficit/Hyperactivity Disorder (ADHD) and Autism Spectrum Disorder (ASD). This expanding body of evidence is increasingly informing the clinical management of individuals with comorbid ADHD and ASD. Areas covered Based on a targeted PubMed search conducted up to March 24,2025,24.03.25, prioritizing meta-analyses or umbrella reviews over primary studies (whenever relevant), with terms encompassing autism, assessment, and treatment, this review addresses: 1) Shared and distinct phenotypic characteristics, neuropsychological features, and genetic and neuroimaging correlates of ADHD and ASD; 2) The assessment of individuals presenting with both ASD and ADHD symptoms; 3) Pharmacological and non-pharmacological strategies for the management for individuals with comorbid ASD and ADHD. Expert opinion The comorbidity of ADHD and ASD should not be overlooked. Nevertheless, before diagnosing comorbid ASD and ADHD, clinicians should perform a thorough differential diagnosis, ensuring that ADHD symptoms are not better explained by ASD. Regarding treatment, further research is warranted to develop personalized approaches, support long-term management strategies, and evaluate real-world outcomes such as quality of life, which are often underrepresented in clinical trials.

Objective:Continuous performance tests (CPT) are often considered the gold standard for the diagnosis attention-deficit/hyperactivity disorder (ADHD), particularly when parent and teacher rating scales are inconclusive. Prior work has indicated that CPT can also help differentiate between ADHD subtypes. However, the ability of CPT to differentiate ADHD subtype has not been examined among youth with comorbid ADHD and anxiety (ADHD+A). This is particularly concerning as the extant literature suggests that anxiety symptoms may exacerbate deficits associated with ADHD (e.g.. , working memory, attention) and attenuate others (e.g., inhibition); thus, anxiety may influence expected patterns on the CPT. This study therefore seeks to examine the role of ADHD subtype on the relationship between ADHD+A and performance on a CPT among youth with ADHD+A.Participants and Methods:Participants included 54 children ranging from 6 to 20 years old (Mage=11.83, 54% female) who were diagnosed with ADHD+A via neuropsychological evaluation. In terms of ADHD subtype, 51.9% (n=28) were diagnosed with ADHD combined or ADHD primarily hyperactive and 48.1% (n=26) were diagnosed with ADHD primarily inattentive. Approximately 46.30% (N=25) of participants were medication naive. Analyses were conducted using data from the Conners Kiddie Continuous Performance Test -Second Edition (KCPT-2), Conners Continuous Performance - Second Edition (CPT-2) and the Conners Continuous Performance - Third Edition (CPT-3), which are part of the same family of performance-based attention measures. Independent samples t-tests were conducted to examine performance differences in aspects of attention (e.g., inattentiveness, sustained attention) and hyperactivity (e.g., impulsivity, inhibition).Results:ADHD subtype was not significantly related to measures of inattentiveness. This includes the number of targets missed (omissions; (t(39)=-.532, p=.59)) and variability in response time (variability; (t(39)=-0.30, p=.77)). In terms of sustained attention, ADHD subtype was not related to variability in response speed across blocks (Hit SEBC/HRT Block Change; (t(39)=-0.26, p=.79)). Importantly, these results were consistent regardless of ADHD medication status. ADHD subtype was also not significantly related to impulsivity. This includes responses to nontargets (commissions; (t(39)=-1.05, p=.30)), random or anticipatory responding (perseverations; (t(39)=-0.19, p=.85)), and mean response speed of correct responses (HR; (t(39)=-0.72, p=.48)).Conclusions:The extant literature suggests that CPT can help clinicians differentiate between ADHD subtypes. However, the results of this study indicate that there are no performance differences on the CPT among youth with comorbid ADHD and anxiety. There are several limitations to consider. First, this study had a relatively small sample size, which also limited the ability to examine ADHD primarily hyperactive/impulsive as a distinct subtype. Additionally, this study did not examine the effect of individual anxiety disorders (i.e., generalized anxiety disorder, specific phobias). Finally, these findings may not generalize to other standardized measures of attention or more ecologically valid measures. Despite these limitations, this study is an important step in understanding the relationship between ADHD+A and performance on attention measures. Clinicians should be cautious in using results from CPT to distinguish between ADHD subtype among children with comorbid anxiety.

Autism and attention deficit hyperactivity disorder (ADHD) comorbidity in the school population have been understudied. This study estimates its prevalence considering both parents' and teachers' reports and clinical diagnosis. Sociodemographic, clinical, and cognitive data were compared by diagnostic groups: autism, ADHD, autism and ADHD, subthreshold autism spectrum disorder (ASD), subthreshold ADHD, and children without neurodevelopmental conditions. Following a two-phase design, 3727 parents and teachers (1802 preschoolers, 1925 school-age children) participated in the first phase. Subsequently, 781 participants underwent individual assessment for DSM-5 diagnoses. The estimated prevalence of the comorbid diagnosis was 0.51% (0.28%-0.74%), with significant sex differences (0.16% girls, 0.89% boys). The cooccurrence of symptoms of autism and ADHD reported by parents or teachers was 3.2% and 2.6%, respectively. ADHD comorbidity was observed in 32.8% of autistic children and 31.4% of those with subthreshold ASD. ASD comorbidity was observed in 9.8% of children with ADHD and 5.7% of those with subthreshold ADHD. Comorbidity was reported by at least one informant in 95% of children. Only 15.8% of children with autism and ADHD had been previously diagnosed with both conditions. Early detection and accurate comorbidity diagnosis are crucial to address the clinical and socio-educational needs of these children.

Over a half of all proteins are glycosylated, and their proper glycosylation is essential for normal function. Unfortunately, because of structural complexity of nonlinear branched glycans and the absence of genetic template for their synthesis, the knowledge about glycans is lagging significantly behind the knowledge about proteins or DNA. Using a recently developed quantitative high throughput glycan analysis method we quantified components of the plasma N-glycome in 99 children with attention-deficit hyperactivity disorder (ADHD), 81 child and 5 adults with autism spectrum disorder, and a total of 340 matching healthy controls. No changes in plasma glycome were found to associate with autism spectrum disorder, but several highly significant associations were observed with ADHD. Further structural analysis of plasma glycans revealed that ADHD is associated with increased antennary fucosylation of biantennary glycans and decreased levels of some complex glycans with three or four antennas. The design of this study prevented any functional conclusions about the observed associations, but specific differences in glycosylation appears to be strongly associated with ADHD and warrants further studies in this direction.

Introduction Substantial overlap exists between autism spectrum disorder and attention deficit hyperactivity disorder at multiple levels. Aetiologically, similarity exists in genetic liability for the two disorders. Phenotypically, comorbidity exists between autism spectrum disorder and symptoms of attention deficit hyperactivity disorder with rates varying between 28% and 92% and both groups have similar problems with social cognition. The aim of this study was to discuss behaviour profiles of children with attention deficit hyperactivity disorder and autism spectrum disorder on the parent PDD Behaviour Inventory. Materials and methods Parent ratings of children with autism spectrum disorder (with and without attention deficit hyperactivity disorder behaviours) were compared with parent ratings of children with attention deficit hyperactivity disorder behaviours who did not have autism spectrum disorder (the attention deficit hyperactivity disorder group) using the PDD Behaviour Inventory, a rating instrument that is agestandardised on children with autism spectrum disorder and which assesses both maladaptive behaviours and adaptive skills. Results The attention deficit hyperactivity disorder group’s maladaptive behaviours were rated as more severe than the autism spectrum disorder group, especially on those domains assessing fears and aggression. Further, the attention deficit hyperactivity disorder group was rated as having greater severity of fears and aggression when compared with those in the autism spectrum disorder group who also had comorbid attention deficit hyperactivity disorder behaviours. By contrast, the attention deficit hyperactivity disorder group was rated as having better expressive language ability than the autism spectrum disorder group (especially when compared with those in the autism spectrum disorder group with comorbid attention deficit hyperactivity disorder behaviours). It is hypothesised that the increased severity of fears and aggression in the attention deficit hyperactivity disorder group may reflect their relatively increased ability to communicate these problems to others. Conclusion These data extend findings from previous studies and suggest that a unique attention deficit hyperactivity disorder profile may exist for children with attention deficit hyperactivity disorder on the PDD Behaviour Inventory.

Background: Autism spectrum disorder (ASD) often co-exists with attention deficit/hyperactivity disorder (ADHD), which may aggravate functional impairment. However, it is unclear how comorbid ADHD symptoms influence the adaptive behavior and social interaction deficits of children with ASD.Methods: The study enrolled 340 children (ranging from 2 to 14 years) with ASD, with comorbid ASD and ADHD, or with typical development (TD). A psychological evaluation involving adaptive behavior and social function was conducted using the Vineland Adaptive Behavior Scale, Second Edition (VABS-II) and the Social Responsiveness Scale (SRS).Results: There was a high prevalence of ADHD symptoms (46.6%) in children with ASD, and children with ASD + ADHD presented the worse profile of ASD symptoms. The ASD + ADHD group had higher scores on VABS and lower scores on SRS in comparison with the ASD alone group and TD group. The regression analysis revealed that ASD symptoms and ADHD symptoms were significantly associated with greater impairments in adaptive behavior and social function. The ADHD symptoms were responsible for an additional 0.8% of the variance in adaptive behavior, and 9.5% of the variance in social function.Conclusions: More severe ASD symptoms and greater impairment in adaptive function and social ability were found in children with ASD and comorbid ADHD, highlighting the need to identify ADHD comorbidities early on in children with ASD and to reduce their negative impact on functioning.

Attention Deficit Hyperactivity Disorder (ADHD) and Autism Spectrum Disorders (ASD) are two of the most represented neurodevelopmental conditions in childhood. The diagnostic shift introduced by the DSM-5, allowing a combined diagnosis of ADHD and ASD, poses different clinical challenges related to diagnostic overshadowing, accuracy of clinical judgment and potential delay in an ASD diagnosis in children presenting with ADHD. Here we tried to disentangle the clinical phenotype and specificity of the two co-occurring conditions in relation to autism traits and empathy, by comparing children with ASD with and without comorbid ADHD with children presenting ADHD only and children with typical development. The child versions of the Autism Quotient (C-AQ) and Empathy Quotient (C-EQ) were administered to a total sample of 198 male children between 6 and 14 years old with age appropriate language skills and normal intelligence. Univariate analysis demonstrated no significant differences in the C-AQ total and subscale scores as well as the C-EQ between children with ASD and children with ASD + ADHD, while children with ADHD alone presented an intermediate phenotype between ASD and TD. Furthermore, a receiver operating characteristic (ROC) analysis was applied to discriminate among the different phenotypes. We found that the C-AQ and C-EQ were accurate at distinguishing with satisfactory reliability between: (a) ASD vs. non- ASD (N-ASD) groups comprising both ADHD and TD children (Area Under the Curve AUC 88% for C-AQ and 81% for C-EQ); (b) ASD and TD (AUC 92% for C-AQ and 95% for C-EQ); (c) ASD and ADHD (AUC 80% for C-AQ and 68% for C-EQ). Our data confirm the reliability of the C-AQ and C-EQ as behavioral markers to differentiate ASD (regardless of comorbid ADHD) from an ADHD condition and TD. Interestingly, in our sample an ADHD condition does not increase the severity of the clinical phenotype in terms of autism traits distribution and empathy, suggesting that the psychological measures detected by the two quantitative instruments are independent of ADHD traits. This evidence will contribute to the translational efforts in developing better tailored treatments and preventive strategies.

Sex differences in the diagnosis of autism spectrum disorder and effects of comorbid mental retardation and attention-deficit hyperactivity disorder

A-164 Processing Speed Differences between Attention-Deficit Hyperactivity Disorder Groups with and without Co-Morbid Anxiety Diagnoses

Attention Deficit Hyperactivity Disorder (ADHD) and Autism Spectrum Disorders (ASD) are two of the most represented neurodevelopmental conditions in childhood. The diagnostic shift introduced by the DSM-5, allowing a combined diagnosis of ADHD and ASD, poses different clinical challenges related to diagnostic overshadowing, accuracy of clinical judgment and potential delay in an ASD diagnosis in children presenting with ADHD. Here we tried to disentangle the clinical phenotype and specificity of the two co-occurring conditions in relation to autism traits and empathy, by comparing children with ASD with and without comorbid ADHD with children presenting ADHD only and children with typical development. The child versions of the Autism Quotient (C-AQ) and Empathy Quotient (C-EQ) were administered to a total sample of 198 male children between 6 and 14 years old with age appropriate language skills and normal intelligence. Univariate analysis demonstrated no significant differences in the C-AQ total and subscale scores as well as the C-EQ between children with ASD and children with ASD+ADHD, while children with ADHD alone presented an intermediate phenotype between ASD and TD. Furthermore, a receiver operating characteristic (ROC) analysis was applied to discriminate among the different phenotypes. We found that the C-AQ and C-EQ were accurate at distinguishing with satisfactory reliability between: a) ASD versus non- ASD (N-ASD) groups comprising both ADHD and TD children (Area Under the Curve AUC 88% for C-AQ and 81% for C-EQ); b) ASD and TD (AUC 92% for C-AQ and 95% for C-EQ); c) ASD and ADHD (AUC 80% for C-AQ and 68% for C-EQ). Our data confirm the reliability of the C-AQ and C-EQ as behavioural markers to differentiate ASD (regardless of comorbid ADHD) from an ADHD condition and TD. Interestingly, in our sample an ADHD condition does not increase the severity of the clinical phenotype in terms of autism traits distribution and empathy, suggesting that the psychological measures detected by the two quantitative instruments are independent of ADHD traits. This evidence will contribute to the translational efforts in developing better tailored treatments and preventive strategies.

Autism spectrum disorder and attention deficit hyperactivity disorder are neurodevelopmental disorders that cannot be codiagnosed under existing diagnostic guidelines (Diagnostic and Statistical Manual of the American Psychiatric Association, 4th ed., text rev.). However, reports are emerging that attention deficit hyperactivity disorder is sometimes comorbid with autism spectrum disorder. In the current study, we examined rates of parent-reported clinically significant symptoms of attention deficit hyperactivity disorder in school-aged children (4-8 years) with autism spectrum disorder, most of whom were first enrolled in our research protocols as toddlers. Results revealed that children with autism spectrum disorder and attention deficit hyperactivity disorder had lower cognitive functioning, more severe social impairment, and greater delays in adaptive functioning than children with autism spectrum disorder only. Implications for clinical practice include the need to assess for attention deficit hyperactivity disorder symptoms at an early age in children diagnosed with autism spectrum disorder. Research is needed to determine efficacious interventions for young children with autism spectrum disorder with comorbid attention deficit hyperactivity disorder to optimize outcomes.