One of the least documented "known" effects of methylphenidate in hyperactive children is the suppression of peer aggression. In this study, 11 aggressive-hyperactive children received a low (0.3 mg/kg) and moderate (0.6 mg/kg) dose of methylphenidate and placebo for 2 weeks each under double-blind conditions. Children were observed in public school settings during classroom seatwork activities, lunch, and recess. Results showed that methylphenidate suppressed nonphysical aggression (p = 0.06) in the classroom, and a moderate dose decreased physical aggression (p less than 0.01) and verbal aggression (p = 0.07) on the playground. The effect on the rate of appropriate social interaction was variable. The majority of subjects exhibited either the same or higher levels of appropriate social interaction on the 0.6 mg/kg dose compared with placebo. In the classroom, both doses of methylphenidate also resulted in reduced levels of motor movement, off-task behavior, noncompliance, and disruptiveness. Teacher ratings of hyperactivity and conduct problem symptoms revealed drug effects, whereas parallel parent instruments did not.