Attention-deficit disorder with hyperactivity is believed, by some, to be a developmental antecedent (predisposing factor) to antisocial personality disorder and criminality. However, evidence supporting this association has not been consistent. We report on a prospective follow-up study of 103 males (ages 16 to 23 years), who were diagnosed as hyperactive (attention-deficit disorder with hyperactivity) between ages 6 and 12 years, and 100 normal controls. The official arrest records of all subjects who resided in New York State during the follow-up interval were obtained. Blind diagnoses (based on structured interviews with subjects and their parents) were made on 98% of the initial cohort at follow-up. Although other investigators have reported on the delinquent behavior of hyperactive children in a prospective design, to our knowledge, follow-up mental status has not been studied previously in relation to official arrest records. Significantly more probands than controls had been arrested (39% vs 20%), convicted (28% vs 11%), and incarcerated (9% vs 1%). The presence of an antisocial/conduct disorder in young adulthood almost completely accounted for the increased risk for criminal activities in the former hyperactive children whether or not it was accompanied by a substance use disorder. Continuing attention-deficit disorder with hyperactivity at follow-up, by itself, was not associated with arrest history. The findings support the view that childhood attention-deficit disorder with hyperactivity is a risk factor for later criminality, but that this relationship is almost exclusively mediated by the development of an antisocial disorder in early adulthood.