Some open-label studies suggest that risperidone can be useful in the treatment of certain target symptoms in children with autism. We aimed to study whether the use of risperidone in comparison with placebo improved functioning in children with autism with regard to behavior (aggressiveness, hyperactivity, irritability), social and emotional responsiveness, and communication skills. We conducted a randomized, double-blind, placebo-controlled trial with 40 consecutive children with autism, whose ages ranged from 2 to 9 years, who were receiving either risperidone or placebo given orally at a dose of 1 mg/day for 6 months. Autism symptoms were monitored periodically. The outcome variables were total scores on the Childhood Autism Rating Scale (CARS) and the Children's Global Assessment Scale (CGAS) after 6 months. Of the 40 children enrolled, 39 completed the trial over a period of 18 months; 19 received risperidone, and 20 received placebo. In the risperidone group, 12 of 19 children showed improvement in the total Childhood Autism Rating Scale score and 17 of 19 children in the Children's Global Assessment Scale score compared with 0 of 20 children for the Childhood Autism Rating Scale score and 2 of 20 children for the Children's Global Assessment Scale score in the placebo group (P < .001 and P = .035, respectively). Risperidone also improved social responsiveness and nonverbal communication and reduced the symptoms of hyperactivity and aggression. Risperidone was associated with increased appetite and a mild weight gain, mild sedation in 20%, and transient dyskinesias in three children. Risperidone improved global functioning and social responsiveness while reducing hyperactivity and aggression in children with autism and was well tolerated. (J Child Neurol 2006;21:450—455; DOI 10.2310/7010.2006.00099).