Intervention programs based on applied behavior analysis (ABA) are currently viewed as the first line treatment for autism spectrum disorder (ASD) in early childhood (Vismara and Rogers 2010). Apart from beneficial effects on a group level, studies have consistently shown large variation in individual gains. Several factors have been suggested to explain this differential response, including child characteristics, intensity of training and level of trainer fidelity to the protocol. Despite scientific evidence, ABA treatment methods as well as goals remain controversial. The structured nature of the intervention program and use of reinforcers can be uncomfortable for parents and trainers in pre-schools. Previous psychotherapy research in adults has shown that therapist allegiance to treatment techniques is linked to the gains achieved (McLeod 2009). It is not unlikely that trainer allegiance to ABA treatment, and thus fidelity to the treatment protocol, is a crucial mediator of ABA efficacy and treatment outcome variation, respectively. To examine the hypothesis that the level of allegiance of trainers is important for ABA treatment gains in preschoolers, a pilot study of 24 children with ASD was conducted. Children were consecutively recruited from a larger ongoing ABA intervention follow-up study in Stockholm county (for details see Fernell et al. 2010) covering all children diagnosed with ASD by psychiatric outpatient departments before the of age 5 years and treated with ABA. Children included in the present pilot-study had received intensive ABA (30 h/week) for 2 years. Two children had been a priori excluded from the sample: the parents of one child declined participation; another child was not included due to a new diagnosis of Rett’s syndrome. Of remaining 24 children (20 boys and 4 girls, age range 2.5–5 years at intake), 19 had autistic disorder, 4 had PDDNOS and one had Asperger’s syndrome. One child had fragile X and one tuberous sclerosis with epilepsy. According to an evaluation of existing test results in records, nine had mental retardation, 12 had uncertain or borderline intellectual functioning, and 3 had normal IQ. Children scored at a mean of 69.9 on the composite score from Vineland Adaptive Behavior Scales (VABS, Sparrow et al. 2005; which includes norm data used) and 49.0 on the Autism Behavior Checklist, above the recommended cutoff of 45 points (Nordin and Gillberg 1996). Thus, this sample may have been representative of the larger group from the follow-up study, who, in turn, can be considered representative of all children receiving an ASD diagnosis in Stockholm (for discussion, see Fernell et al. 2010). For the assessment of treatment outcome, the children’s parents completed the VABS pre and post 2 years of intensive ABA treatment. The 24 children all had one preschool trainer each. These trainers were all employed at the kindergartens and worked exclusively with the child in the study. They were L. Klintwall (&) Akershus University College, P.O.Box 423, 2001 Lillestrom, Norway e-mail: lars.klintwall@hiak.no