HYPOTHESES: (1) Risperidone will be more effective than placebo in reducing impulsive aggression, agitation, self-injurious behavior, and troublesome repetitive behavior associated with autism. (2) Risperidone will result in more sedation (transient) and weight gain than placebo. (3) Patients continued on risperidone will be significantly less likely to experience exacerbation of symptoms of irritability, aggression, agitation, and stereotypy than those randomized to placebo, as measured by the Aberrant Behavior Checklist (ABC), the Ritvo-Freeman Real Life Rating Scale, and the compulsions scale from the Children's Yale-Brown Obsessive Compulsive Scale (CY-BOCS). (4) Patients continued on risperidone would show superior adjustment and functioning at the end of the trial, as evidenced by lower Clinical Global Impression ratings, when compared to patients randomized to placebo.