This study is designed to determine the effectiveness of risperidone, a drug treatment for the interfering symptoms of Autistic Disorder in children and adolescents between the ages of 5 and 17. Between 100 and 120 patients will be participating in this research study at five academic medical centers in the United States. The primary aim of the treatment is to reduce impairing behavioral symptoms such as aggression, explosive outbursts, or self-injurious behavior, without significant side effects. A secondary aim is to evaluate possible improvement in the level of social relatedness, attention, motor coordination, and short-term memory.