Conduct and psychiatric disorders are found among a higher proportion of people with mental retardation than among people who are not mentally retarded. Among many different treatment approaches to conduct disorder are drug therapy, behavioral treatment, psychotherapy, cognitive and social learning. Studies have suggested that neuroleptic drugs, such as risperidone, may be beneficial in treating conduct disorder in mental retardation. This is a randomized, double-blind study to evaluate the effectiveness of risperidone compared with placebo in the treatment of children 5 to 12 years of age with mild, moderate, or borderline mental retardation who display destructive behaviors. The study has 2 phases: a run-in phase of 1 week and a treatment phase of 6 weeks. Patients receive placebo to be taken orally once a day during the first week (run-in). On the basis of scores on the Nisonger Child Behavior Rating Form (N-CBRF) after the first week, patients either continue in the double-blind treatment phase or discontinue the study. During the treatment phase patients receive risperidone oral solution once daily at a starting dose of 0.01 mg/kg body weight, increasing gradually at the investigator's discretion up to 0.06 mg/kg (maximum), or placebo for 6 weeks. A parent or caregiver evaluates the child's behavior and symptoms at scheduled office visits during the course of treatment. The primary measure of efficacy is the change from baseline to the end of treatment in the Conduct Problem subscale of the N-CBRF. Other efficacy assessments include the changes in the Aberrant Behavior Checklist (ABC), Behavioral Problems Inventory (BPI), and the Clinical Global Impression (CGI), a rating system used to evaluate the overall and severity of clinical change. Safety assessments include the incidence of adverse events throughout the study; weekly measurement of vital signs (pulse, temperature, blood pressure) and evaluation of the presence and severity of extrapyramidal symptoms by the Extrapyramidal Symptom Rating Scale (ESRS); and clinical laboratory tests (hematology, biochemistry, urinalysis) before study initiation and at end of treatment. The study hypothesis is that risperidone will be well tolerated and effective for the treatment of conduct disorder in children aged 5 to 12 years with mild, moderate, or borderline mental retardation. Risperidone oral solution 1.0 mg/mL once daily. Days 1 and 2 at a dose of 0.01 mg/kg body weight, Day 3 at a dose of 0.02 mg/kg, and increasing gradually up to 0.06 mg/kg (maximum) daily through 6 weeks. Dose may be increased or decreased at investigator's discretion