This study was designed to explore the placebo-controlled effects of risperidone on cognitive-motor processes, dyskinetic movements, and behavior in children receiving maintenance risperidone therapy. Sixteen children aged 4–14 years with disruptive behavior were randomly assigned to drug order in a crossover study of risperidone and placebo for 2 weeks each. Dependent measures included tests of sustained attention, memory, visual matching, tremor, seat activity, abnormal movements, and parent behavior ratings. Results were compared by repeated measures ANOVA. Fourteen boys and 2 girls with disruptive behavior and IQ≤84 all completed the protocol. Risperidone was superior to placebo on response time (p =0.01, η P 2 =0.43) and seat movement (p <0.05, η P 2 =0.29) on a short-term memory task, and on a measure of static tremor (p =0.05, η P 2 =0.28). There was not a significant difference between treatment conditions on the Abnormal Involuntary Movement scale. Risperidone was superior to placebo on three subscales of the Nisonger Child Behavior Rating Form [Overly Sensitive (p <0.01, η P 2 =0.44), Conduct Problem (p =0.02, η P 2 =0.36), Hyperactivity (p =0.03, η P 2 =0.32)] and on the Hyperactivity/Noncompliance subscale of the Aberrant Behavior Checklist (p =0.01, η P 2 =0.41). Significant increases in heart rate (p =0.05, η P 2 =0.27) and weight (p =0.02, η P 2 =0.36) occurred in the risperidone condition. The findings suggest a beneficial effect of risperidone after several months of treatment on efficiency of responding, activity level, static tremor, and aspects of behavior.