We gave increasing daily doses of clozapine to six patients with advanced Parkinson's disease (PD) and levodopa-induced dyskinesias. Clozapine reduced the daily dyskinesia time five-fold, increased "on" time eight-fold, and doubled the serum [DOPA] producing half-maximal dyskinesia. Parkinsonism scores after overnight DOPA withdrawal improved with increasing daily clozapine intake, and there was no clozapine dose-related shift in levodopa dose response for relief of parkinsonism. Patients experienced sedation, sialorrhea, and orthostatic hypotension. Clozapine appears to be an effective agent for suppression of levodopa-induced dyskinesias in PD.