This study reports on the psychiatric outcome as young adults (ages 16-23) of 50 hyperactive boys compared with their nonhyperactive brothers and to unrelated controls. Siblings provide a built-in control for potentially important factors, such as social class and parental psychopathology. Blind assessments at follow-up showed that significantly more probands (30%) than siblings (6%) and controls (10%) had multiple DSM-III diagnoses. Moreover, probands had a more severe form of antisocial disorder than siblings. In conclusion, the increased risk for multiple and more pervasive mental disorder in the young adulthood of hyperactive boys cannot be attributed to childhood factors shared with siblings.