This report describes the use of risperidone in the treatment of two very young children with autistic disorder, a 29-month-old boy and a 23-month-old boy, respectively. These children presented with severe and persistent symptoms of aggression and irritability that had not responded to previous treatment. In both cases, risperidone significantly reduced aggression and improved social relatedness. One patient's treatment with risperidone was complicated by persistent tachycardia and QTc interval prolongation that was dose-related. Consideration should be given to the appropriate use of medication in the treatment of very young children with autism when other interventions do not prove helpful.