In the first study, 80 hyperactive boys -- who were referred to treatment in 1970-1972, were treated with stimulant medication and followed up for a mean of 9.3 years. In the second, 50 hyperactive boys received multimodal therapy (MMT) consisting of medication, parent psychotherapy, and intensive individual psychotherapy with the child, and were followed up for a mean of 8.7 years. The groups were well-matched on pretreatment variables. Official teenage arrest rates and institutional rates were significantly lower for the MMT group. These intergroup differences were attributable to an MMT subgroup who continued longer in treatment and who were found to be significantly improved at 1-, 2-, and 3-year followups. These findings highlight the importance of long-term evaluations in treatment studies of chronic, handicapping childhood disorders and suggest that MMT in childhood is a cost-effective approach to delinquency prevention in hyperactive boys. 4 tables and 20 references. (Author abstract modified)