Objective: To evaluate the effects of risperidone augmentation for treatment-resistant aggression in children with attention-deficit/hyperactivity disorder (ADHD). Method: Twenty-five children (ages 7-12 years) with attention-deficit/hyperactivity disorder(ADHD) and significant aggressive behaviors were randomized to risperidone or placebo for 4 weeks for this double-blind study. Subjects were already in treatment with a constant dose of psychostimulant medication. The primary efficacy measure was change from baseline in the Children's Aggression Scale-Parent (CAS-P) and -Teacher (CAS-T) total scores. Results: The mean risperidone dose at endpoint was 1.08 mg/day. For the CAS-P total score, a significant difference was found ([x.sub.1.sup.2] = 4.30, p less than 0.05) with 100% of risperidone subjects improving by more than 30% from baseline to endpoint, whereas only 77% of the placebo group reported a similar response. No differences were found on the CAS-T total score. For the CAS-P and CAS-T, no significant interaction was found between treatment group and time. Rates of adverse events did not differ significantly between groups. Conclusions: Risperidone treatment appears to be well tolerated and modestly effective when used in combination with psychostimulants for treatment-resistant aggression in children with ADHD. (Contains 2 figures and 3 tables.)