The relationship between patients with attention deficit hyperactivity disorder (ADHD) and those with pervasive developmental disorders (PDD) was studied at Okayama University Graduate School of Medicine, Japan. Using the High-Functioning Autism Spectrum Screening Questionnaire (ASSQ) and ADHD Rating Scale-IV, the ASSQ scores of the PDD group (9 boys and 6 girls; 5-13 years) and ADHD group (19 boys and 1 girl; 5-15 years) were significantly higher than the control normal group (38 boys and 2 girls; 5-15 years); and the PDD group scored higher than the ADHD group. The WISC-III Full Scale IQ was average and similar in the 2 test groups. Both groups had higher scores than controls in each of the three behavioral disorders on the ASSQ: reciprocal social interaction, verbal and nonverbal communication and imaginative activity, and restricted and repetitive behavior. The PDD and ADHD group had similar scores in communication problems and restricted and repetitive behavior. PDD patients had a higher score than the ADHD group only in the social interaction impairment. In total score, inattention score, and hyperactivity/impulsivity score on the ADHD Rating Scale-IV, both groups were significantly higher than the control group; ADHD and PDD groups showed similar scores in the three ADHD subtypes. ADHD patients had many PDD-related symptoms, and PDD patients had many ADHD-related symptoms. Diagnoses of ADHD and PDD were difficult to distinguish when using the DSM-IV criteria, and an individual patient should be evaluated using both questionnaires. 
COMMENT. Although patients with PDD, according to DSM-IV criteria, were excluded from the ADHD group, the ASSQ total score in the ADHD group was higher than in the control group. Also, all three behavioral category scores in the PDD diagnosis (social interaction, communication problem, and restricted and repetitive behavior) were higher in the ADHD group than in the control group. The ADHD group included subjects whose PDD-related symptoms did not reach the threshold for PDD diagnosis, based on DSM criteria. Social interaction was the only domain in PDD criteria that was not significantly impaired in the ADHD group, possibly because at least 2 items are required whereas one will suffice in the other domains. In our ADD Neurology Clinic at Children's Memorial Hospital, Chicago, we also have been impressed with the prevalence of symptoms of autism complicating ADHD cases. Sometimes, these symptoms are only apparent after completing a simplified questionnaire for ASD with the parent, leading to more adequate and individualized behavioral and educational intervention. The reported prevalence of autistic spectrum disorder has increased from as few as 4 per 10,000 children in the 1980s to as many as 50 per 10,000 today.