We studied the relationship between patients with attention-deficit/hyperactivity disorder (ADHD) and those with pervasive developmental disorders (PDD), using the High-Functioning Autism Spectrum Screening Questionnaire (ASSQ) and ADHD Rating Scale-IV. The ASSQ scores of the PDD group and the ADHD group were significantly higher than the control group. Furthermore, the PDD group scored higher than the ADHD group. Both groups also showed higher scores than the control group in all three domains, that is, restricted and repetitive behavior, social interaction, and communication problem. The PDD and the ADHD group showed no significant difference in the domains of communication problem, and restricted and repetitive behavior. The PDD group had a higher score than the ADHD group only in the social interaction domain. In total score, inattention score, and hyperactivity/impulsivity score on the ADHD Rating Scale-IV, both groups were significantly higher than the control group. Between the ADHD and the PDD groups, there was no significant difference in the three scores. The patients with strictly diagnosed ADHD had many PDD-related symptoms, and the patients with PDD had many ADHD-related symptoms. It therefore seems difficult to make a distinction between ADHD and PDD by using the present diagnostic criteria in the DSM-IV. We should evaluate each patient in terms of both sets of criteria.