The effect of methylphenidate on aggression in adolescents diagnosed with both aggressive conduct disorder and attention deficit disorder with hyperactivity was assessed in nine male adolescents. After three open trials, a placebo controlled double-blind design was used. During methylphenidate treatment of the six double-blind subjects, there was a significant reduction of aggressivity (p's less than 0.05), as measured by the Adolescent Antisocial Behavior Checklist. Conners Teacher Rating Scale Hyperactivity and Aggression scores were in the predicted directions, but the differences were not statistically significant.