Compared to controls, the occurrence of ADHD was found to be related to the male gender and to the occurrence of ADHD-related symptoms in siblings (odds ratio: 13.50). Birth order and MR were not associated with the occurrence of ADHD and ADHD-related symptoms. ADHD- related symptoms increased if a further sibling also suffered from ADHD. Conclusions. Among a sample of Iranian children suffering from ADHD, the ADHD and ADHD-related symptoms in childhood were found to be related to the male gender and to the occurrence of ADHD in siblings. Moreover, birth order was found to be unrelated. The fact that symptoms of ADHD-related symptoms increased if a further sibling was suffering from ADHD, and decreased if a further sibling was suffering from MR, is intriguing and needs further explanation.