--Disease Characteristics-- Presumed idiopathic Parkinson's disease Presence of three of the cardinal features: Rest tremor Rigidity Bradykinesias/akinesia Postural and balance abnormalities Absence of alternative explanations for the syndrome Absence of atypical features Psychosis of at least 4 weeks duration requiring treatment --Prior/Concurrent Therapy-- Chemotherapy: No concurrent chemotherapy Other: No use of any dopamine blocking drug within the past 3 months No use of depot neuroleptic within the past 12 months No change of antidepressant or anxiolytic dose within the past 1 month No prior clozapine for psychosis Anti-PD medications stable for at least 7 days before study entry --Patient Characteristics-- Hematopoietic: No history of leukopenia No active blood dyscrasia other than mild anemia Renal: No active problems with urinary retention Cardiovascular: No symptomatic orthostatic hypotension No uncontrolled angina No myocardial infarction within the past 3 months Other: Fertile patients must use effective contraception No uncontrolled seizures (i.e., 1 or more seizures per month over the past 6 months) No dementia precluding accurate assessment on psychiatric assessment battery No AIDS No other illness that would make use of clozapine potentially hazardous No narrow angle glaucoma No systemic factor contributing to the psychosis (e.g., urinary infection, liver disease, renal failure, anemia, infection, etc.)