Attention deficit–hyperactivity disorder (ADHD) has been noted for its high rate of comorbidity. The present study is the first report in Japan evaluating the proportion of comorbidity in ADHD cases presenting in the clinical setting, aiming at clarifying the picture of ADHD in Japan. The subjects consisted of 68 child and adolescent cases meeting criteria for ADHD (Diagnostic and Statistical Manual of Mental Disorders, 4th edn) under treatment at a child psychiatry clinic (IQ > 50, mental age ≥ 4 years old). Disorders evaluated as comorbid disorders were mood disorders, anxiety disorders, elimination disorders, sleep disorders, tic disorders, oppositional defiant disorder (ODD), conduct disorder (CD), school refusal, and epilepsy. Comorbidity with mood disorders, anxiety disorders, ODD, and CD, were found to be lower than the high rates conventionally reported in North America. The lower age of the present subjects, primarily in infancy and elementary school age with few adolescent cases, and a bias towards milder cases from an outpatient clinic without inpatient facilities are believed to be factors accounting for this disparity. Furthermore, it was a notable fact that mentally delayed cases (IQ: 51–84) amounted to 34% of the cases, indicating the necessity to consider intelligence level when formulating a treatment strategy for ADHD.