Abstract Background The HEADS-ED screening and triage tool was developed for children 6 years to young adults as a communimetric mental health (MH) and addictions screener to help clinicians identify and communicate areas of need and recommended clinical care. Few clinician-administered tools identify both MH and developmental needs in young children under 6. From a review of milestone tools and in consultation with paediatricians, psychiatrists, and family physicians, the HEADS-ED Under 6 is a MH and developmental screening and triage tool that was developed to screen and flag functional impairment and action required in several areas, including: Home & caregivers, Eating & sleeping, Activities & peers, Development /speech/language/motor, Safety, Emotions & behaviours and Discharge & current resources (see http://heads-ed.com for more background and access to both tools available in English and French). Objectives Our objectives were to examine: 1) feasibility of implementing the new HEADS-ED Under 6 screening and triage tool in community and hospital settings, 2) clinical utility of the tool to assist with clinical decision making, and 3) validity of the tool against a comprehensive assessment. Design/Methods We conducted two implementation and validation studies to examine uptake of the tool and how the tool assists in clinical referral decisions to more specialized and intensive services at 2 sites. For our initial validation study, we piloted and validated the tool at a community MH agency in Ontario for children under the age of 6 from November 2019- March 2021 (N=566). For our cross-validation study, we examined how the tool was implemented within a regional coordinated access service housed in a paediatric hospital in Ontario from June 2021- August 2023 (N=589). Results For our initial validation study, the HEADS-ED Under 6 was widely used by intake workers (95%, N = 536/566) to communicate children’s MH and developmental needs (Figure 1) and assisted in recommending services of varying intensity. The tool showed good concordance with the InterRAI Early Years assessment in similar domains requiring no action (61.1% to 81.6%), requiring action but not immediate (49.2% to 88.0%) and requiring immediate action (66.7% to 100%) for children under 4 years old (p's <.001). Three clinical domains (Eating & sleeping, Development/speech/language/motor, and Emotions & behaviours) also independently predicted an urgent care recommendation (p's < .001). For our cross-validation study, the tool was used for all 589 intake appointments, and 96.4% (N = 568/589) had a level of need documented. HEADS-ED Under 6 clinical domains (.24-.38) and total scores (.52) were significantly correlated with level of need (p<.001). Higher HEADS-ED Under 6 total scores helped intake workers decide on more intense services based on level of need (p<.001; Figure 2). ROC analyses (AUC = .807, SE = 68.5% sensitivity, 79.5% specificity) confirmed a total score of 6 or above helped triage to more targeted and intense services. Conclusion The HEADS-ED Under 6 is a brief, straight-forward, and valid screening and triage tool that can be used by intake workers, physicians, and paediatricians to guide their interview in several areas of MH and development, communicate the severity of the child’s MH and developmental needs, and assist with determining level of need for services. When a HEADS-ED Under 6 total score reaches ≥6, clinicians should advocate for more intensive services when making MH service referrals.