Many autistic patients with mental retardation have difficulties with explosivity and aggression. They often prove resistant to various pharmacotherapeutic interventions. In this study, 11 male outpatients (mean 18.3 years) were administered risperidone in an open-label fashion. The risperidone was started at 0.5 mg daily, and titrated upwards until maximum clinical benefit occurred. Serial clinical interviews were conducted, and Conners Parent-Teacher Questionnaires (short form) were completed by the caretakers. Substantial clinical improvement was noted almost immediately in each patient, with aggression, self-injury, explosivity, and poor sleep hygiene most improved. The modal dose for optimal response was 0.5 mg bid. Weight gain was a significant side effect (average velocity of 0.47 kg per week), while none of the patients experienced extrapyramidal side effects.