A double-blind, placebo-controlled trial was performed to determine the efficacy and tolerability of 8 weeks of treatment with risperidone in the management of 48 adolescent and adult patients with Tourette syndrome. Twenty-four patients were randomly assigned to treatment with risperidone in doses of 0.5 to 6.0 mg/day, and 24 were assigned to placebo. The dosage of medication was increased in fixed increments during the first week of double-blind treatment and thereafter in a flexible dose regimen according to clinical response. Risperidone, at a median dose of 2.5 mg/day (range, 1 to 6 mg/day), was found to be significantly ( p < 0.05) superior to placebo on the Global Severity Rating of the Tourette Syndrome Severity Scale. The proportion of patients who improved by at least one point on this seven-point scale was 60.8% in the risperidone group and 26.1% in the placebo group. Treatment with risperidone was accompanied by an improvement in global functioning in patients with average to above-average impairment at baseline as measured by the Global Assessment of Functioning scale. With respect to extrapyramidal symptom scores measured on the Extrapyramidal Symptom Rating Scale, hypokinesia and tremor increased in the risperidone group, but the effect on tremor was largely confined to subjects with higher baseline tremor scores. There were no significant differences in dystonic reactions, dyskinetic movements, subjective parkinsonism, or akathisia. Risperidone did not increase obsessive-compulsive symptoms. Fatigue and somnolence were the most common adverse events associated with risperidone.