SUMMARY
Aim: To estimate the prevalence of attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD) and conduct disorder (CD) in children and adolescents previously diagnosed with autism spectrum disorder (ASD). Methods: A cross-sectional study involving 71 children and adolescents previously diagnosed by a psychiatrist as having ASD. All were enrolled at a school that accompanies only individuals with ASD. The evaluation instruments consisted of a sociodemographic questionnaire and the Brazilian version of the semi-structured interview Kiddie-SADS-PL for the investigation of psychiatric disorders in children and adolescents. Results: Overall, 62% of the children in the sample had some psychiatric comorbidity: 49.3% had some form of ADHD, 11.3% ODD and 1.4% CD. In relation to the children with ADHD, 11.3% fulfilled the criteria for the inattentive subtype, 12.7% for the hyperactive/impulsive subtype, 7.1% for the combined subtype and 18.3% for ADHD not otherwise specified. Conclusion: These results ratify the clinical heterogeneity of ASD and highlight the importance of diagnosing comorbidities that could affect the clinical status and functioning level of children and adolescents with ASD.