We treated 17 patients with Parkinson's disease (PD) complicated by psychosis with the atypical antipsychotic drug, clozapine, for 6 to 24 months (mean, 15 months) in a prospective, open-label trial. At 3-month intervals we evaluated patients, using a simplified brief Psychiatric Rating Scale (PRS), the motor examination portion of the Unified Parkinson's Disease Rating Scale, and the Mini-Mental State Examination (MMSE). Mean PRS score was significantly improved when compared with baseline over 1 year (p < 0.01) and nonsignificantly improved for the second year. We maintained the levodopa dose at levels that were 17 to 68% higher than baseline, and the mean motor examination score improved by 11 to 22% in the first 15 months. Clozapine dosage utilized in the trial ranged from 6.25 mg every other day to 150 mg/d. Adverse effects, including sedation and confusion, were common. These results demonstrate that clozapine therapy can be effective in treating psychosis in PD patients over 1 to 2 years. The decline in efficacy in the second year was most likely related to an increase in daily levodopa dose, progression of dementia (illustrated by a decline in MMSE score), and an inability of PD patients to tolerate higher doses of clozapine.