Objective To examine the relationship between attention deficit disorder with hyperactivity in childhood and criminality in adolescence and adulthood in 89 hyperactive and 87 normal control subjects. Method In this prospective study, adolescent follow-up intervals ranged from 13 to 21 years and adult follow-up ranged from 18 to 23 years. The official arrest records for all subjects were obtained. Results Hyperactive subjects had significantly higher juvenile (46% versus 11%) and adult (21% versus 1%) arrest rates. Juvenile and adult incarceration rates were also significantly higher. Childhood conduct problems predicted later criminality, and serious antisocial behavior in adolescence predicted adult criminality. Conclusions Hyperactive children are at risk for both juvenile and adult criminality. The risk for becoming an adult offender is associated with conduct problems in childhood and serious antisocial behavior (repeat offending) in adolescence. Hyperactive children who do not have conduct problems are not at increased risk for later criminality.