Although the DSM-IV diagnostic criteria for Attention Deficit/Hyperactivity Disorder (AD/HD) exclude Pervasive Developmental Disorder (PDD), some clinicians find that the two disorders can be comorbid and, in fact, make a dual diagnosis. Nevertheless, few empirical studies have investigated the clinical necessity for this practice. In the first of our two studies, children with high-functioning PDD were selected from among 520 outpatients. Of these, children also meeting the DSM-IV criteria for AD/HD were identified through a psychologist's observation, the completion of the ADHD-Rating Scale by parents and/or teachers, and a child psychiatrist's examination. We then examined the impact of PDD subtype and age on the co-occurrence rate. Study 2 analyzed comorbidity in two cases taken from Study 1. Of the 53 subjects in Study 1, 36 children also met the DSM-IV criteria for AD/HD. The co-occurrence rate for Asperger's Disorder (AS)/Pervasive Developmental Disorder, Not Otherwise Specified (PDDNOS) (85%) was significantly higher than for Autistic Disorder (57.6 %), and AD/HD symptoms were more common in younger children. Study 2 demonstrated the existence of comorbidity of PDD and AD/HD as separate disorders. We conclude not only that AD/HD symptoms occur frequently in children with PDD, but also that in some cases a dual diagnosis is essential to the implementation of effective treatment.