Compared with the local normative comparison group (n=272), children who participated in the Multimodal Treatment Study of Children With ADHD (MTA; n=487) had significantly higher rates of delinquency at 36 months (27.1 percent for the MTA children compared with 7.4 percent for comparison children). The MTA children also had higher rates of substance use (17.4 percent compared to 7.8 percent at 36 months). MTA children randomly assigned to intensive behavior therapy reported less 24-month substance use than other MTA children. At 24 and 36 months, more days of prescribed medication were associated with more serious delinquency but not substance use. The cause-and-effect links between medication treatment and delinquency are unclear. The absence of associations between medication treatment and substance use should be reassessed at older ages. These findings show the need to continuously monitor treatment outcomes as children with ADHD enter adolescence. Each of 6sites randomly selected 96 to 98 children to 1 of 4 treatment groups: intensive multicomponent behavior therapy, intensive medication management, a combination of the 2 treatments, or referral to traditional community care. Participants were reassessed at completion of the 14-month treatment phase (ages 8-12), at 24 months, and at 36 months. 3 tables and 65 references