We undertook a systematic review of the literature on the long-term treatment of attention-deficit hyperactivity disorder (ADHD). We used systematic strategies to identify randomized treatment studies in which treatment was administered for 12 weeks or more. We included 14 studies involving 1379 subjects. Because of the limited number of high-quality studies and the heterogeneity of outcome measures, we did not perform meta-analysis. We rated 5 studies as adequate for methodological quality. Five studies followed children for more than 26 weeks. Pharmacologic interventions were studied more frequently than non-pharmacologic ones. Six studies permitted evaluation of the effects of combined drug and behavioural intervention. Twenty-five different outcomes were measured using 26 different tests. Stimulant medication appears to reduce ADHD (7 studies), dysfunctional social behaviour (6 studies), and internalizing symptoms (2 studies). Available studies provide little evidence for improved academic performance with stimulants (3 studies). Medications other than stimulants have not been studied extensively (3 studies). Only 1 study showed that combination therapy adds to the effects of medication. Rigorous treatment research among representative samples of ADHD individuals is needed.