Autism Spectrum Disorders (ASD) are highly heterogeneous disorders. The heterogeneity of Autistic Spectrum Disorders can be determined by comorbid psychiatric disorders like ADHD (Attention-Deficit/Hyperactivity Disorder). The objective of this research is to analyse the outcome of a sample of preschoolers diagnosed with Autism Spectrum Disorder (ASD) comorbid with ADHD symptoms involved in an Applied Behavioural Analysis program and to compare the efficacy of ABA between this group and a control group comprised of children with ASD alone. We monitored, using ADOS (Autism Diagnostic Observation Schedule) and ADHD -RS (ADHD Rating Scale) at the baseline (T0) and after one year (T1), a sample of 48 children (2-5 years old) diagnosed with ASD and with ADHD symptoms. In this period the children follow an applied behavioural analysis program (2 hours / day, 5 days / week for 1 year). After one year of applied behavioural analysis, the improvement in ADOS scores in the control group was significantly higher than for those in the study group (F=9.08, p=.00, partial Eta squared = .82). [ABSTRACT FROM AUTHOR]