Results of two prospective longitudinal studies of predelinquent hyperactive boys are presented. The first was a study of 80 Caucasian hyperactive boys treated with stimulant medication and the second a study of 50 Caucasian hyperactive boys treated with multimodality treatment (MMT). The two groups were well matched on before-treatment variables. Official teenage arrest and institutional rates were found to be significantly lower in the multimodality treated group. These findings demonstrate the importance of long-term evaluations in treatment studies of chronic handicapping childhood disorders and suggest that MMT in childhood is a cost-effective treatment approach for delinquency prevention in hyperactive boys.