Background: Risperidone is an atypical anti-psychotic drug that blocks dopamine as well as serotonin receptor systems. The present study was designed to examine the efficacy and safety of risperidone in a 6-week double-blind, randomized, parallel-group design in the treatment of aggression in adolescents with a primary diagnosis of DSM-IV disruptive behavior disorders and with subaverage intelligence. Method: We randomly assigned 38 adolescents (33 boys; 10 subjects with slightly subaverage IQ, 14 with borderline IQ, and 14 with mild mental retardation), who were hospitalized for treatment of psychiatric disorders associated with severe aggression, to receive risperidone or placebo. The main efficacy measures were the Clinical Global Impressions-Severity of Illness scale (CGI-S), the modified Overt Aggression Scale (OAS-M), and the Aberrant Behavior Checklist (ABC). Side effects were measured using the Extrapyramidal Symptom Rating Scale (ESRS). Results: The mean daily dose of risperidone at the end of treatment was 2.9 mg (range, 1.5-4 mg). Risperidone, compared with placebo, was associated with significant improvements on the CGI-S (p < .001) and the at-school ABC overall and hyperactivity scales (p < .05). During a 2-week washout following the 6-week trial, a statistically significant worsening was found in the risperidone group on the CGI-S scale, the OAS-M, and the ABC. Extrapyramidal symptoms were absent or very mild during risperidone treatment. Transient tiredness was present in 11 (58%) of 19 drug-treated subjects. Other untoward effects included sialorrhea, nausea, and slight weight gain (mean = 3.5% of body weight in the risperidone group). No clinically relevant changes were found in laboratory parameters, electrocardiogram, hear rate, or blood pressure. Conclusion: These results suggest that risperidone may be effective for severe aggression in adolescents with disruptive behavior disorders and subaverage intelligence, and these results are consistent with reports suggesting its effectiveness for treating severe aggression in adolescents in general.