Risperidone has shown safety and efficacy for aggressive and destructive behaviors in short-term studies. This longer-duration study includes a broad sample. Forty subjects, aged 8–56 years (mean=22), all with mental retardation and 36 with autism spectrum disorders participated in this 22-week crossover study, with 24 weeks of open maintenance thereafter. Of 40 subjects, 23 (57.5%) responded fully (50% decrease in Aberrant Behavior Checklist-Community Irritability subscale score), while 35 subjects (87.5%) showed a 25% decrease. Gender, mood disorder, and antiseizure medications did not alter response. Increased appetite and weight gain were common. Low dose risperidone was effective for aggressive behavior in persons with MR. More long-term studies are needed, incorporating weight control interventions.