We previously reported a prospective follow-up study of 101 young adult males whose conditions had been diagnosed as hyperactivity in childhood. Compared with controls, probands had significantly higher rates of attention-deficit, antisocial, and drug use disorders at follow-up (mean age, 18 years). The present study was an attempt to replicate these findings on an independent sample of 94 hyperactive boys who were seen at the same clinic, compared with 78 normal controls. Assessments were made by clinicians who were blind to group membership. Information was obtained for 90% of the original cohort. As in the previous study, significantly more probands than controls were given ongoing diagnoses of attention-deficit disorder (43% vs 4%), antisocial disorders (32% vs 8%), and drug use disorders (10% vs 1%). Furthermore, the absolute rates of these disorders were comparable for corresponding groups across studies, and the adjusted odds ratios did not differ significantly. As previously, there was no increased risk for affective disorders in the grown hyperactive children. The present study provides a powerful replication of the nature of the young adult outcome of childhood hyperactivity.