The records of 49 patients with Parkinson's disease and psychosis who were treated with clozapine for up to 18 months were reviewed. Average starting dose of clozapine was 16 mg. Average maximum dose was 39 mg. The psychotic symptoms improved in 76% of the patients at 3 months, and response to clozapine within the first year ranged from 71% to 80%. This response allowed a maximization of levodopa dose. Improvements in scores on the Unified Parkinson's Disease Rating Scale and tremor subscale were seen in some patients but were not statistically significant. This study, the largest of its kind to date, suggests that clozapine is well tolerated and effective in treating psychosis in patients with Parkinson's disease.