Currently, both the DSM-IV-TR and ICD-10 preclude the diagnosis of attention deficit/hyperactivity disorder (ADHD) in cases that present with an autism spectrum disorder (ASD). This criterion will be removed in the upcoming DSM-V, but the relationship between ASD and ADHD, and in particular the prevalence of ADHD among the ASD population, remains controversial. Previous studies have reported clinically significant ADHD symptoms in one-third to three-quarters of ASD-affected individuals (probands). In our sample of 1,838 simplex children and adolescents with ASD, we found that less than 16% met clinically significant levels of ADHD symptoms, per parent report. When both parent and teacher reports were considered, the comorbidity rate was even lower, at 2%.