Given the importance of interpersonal interactions for hyperactive children, we evaluated the impact of methylphenidate on specific categories of social behavior in 25 boys, aged 6-12, with attention deficit-hyperactivity disorder (ADHD). These children participated in a 3-week, double-blind, crossover trial with placebo and low (0.3 mg/kg) and moderate (0.6 mg/kg) dosages of methylphenidate during a naturalistic summer research program. Fifteen comparison boys, without problems in attention and behavior, were also observed. In addition to decreasing noncompliance, methylphenidate reduced a combined category of physical and verbal aggression for the ADHD boys, with a significant linear trend across dosages. The medication decreased aggression to levels comparable with those of the comparison boys. There were no medication effects on the frequency of nonsocial or prosocial behaviors. Results are discussed in light of the need to effect durable change in both the quantity and quality of social behavior for hyperactive children.