Objective: To determine the long-term safety and effectiveness of risperidone for severe disruptive behaviors in children. Method: A multisite, 1-year, open-label study of patients aged 5 to 14 years with disruptive behaviors and subaverage intelligence was conducted. Results: Seventy-three percent of the 504 patients enrolled completed the study. The mean [+ or -] SE dose of risperidone was 1.6 [+ or -] 0.0 mg/day. The most common adverse events were somnolence (30%), rhinitis (27%), and headache (22%). The incidence of movement disorders was low, and mean Extrapyramidal Symptom Rating Scale scores decreased during risperidone treatment. No clinically significant changes in mean laboratory values were noted, except for a transient increase in serum prolactin levels. Scores on the Nisonger Child Behavior Rating Form Conduct Problem Scale improved significantly as early as early as week 1, and improvement was maintained throughout the trial (p Less than .001 at each time point). Significant improvements were noted on positive social behavior and other Nisonger Child Behavior Rating Form subscales, Aberrant Behavior Checklist, Clinical Global Impressions scale, and tests of patients' cognitive function (each p less than.001). Conclusions: Risperidone was well tolerated and effective in the long-term treatment of disruptive behavior disorders in children with subaverage intelligence.