Risperidone is an atypical antipsychotic agent with dopamine and serotonin antagonistic effects. It is an effective treatment for reducing aggressive behavior in adults with mental retardation. The use of risperidone in a severely mental retarded child with aggression is described. Risperidone was able to reduce the aggressive behavior in this patient. No serious side effect was found. This case illustrated that risperidone is effective and well tolerated in treating aggressive behavior in children with severe mental retardation. However, the anti-aggressive effect of risperidone in mentally retarded children remains to be seen in a larger sample-size study.