Twenty-nine elderly patients who failed treatment with clozapine, risperidone, or olanzapine entered this 24-week, single-center, open-label trial to assess the efficacy of quetiapine (12.5-400 mg/day) for psychosis in patients with Parkinson's disease (PD). Psychiatric, motor, and cognitive assessments were administered at baseline and at periodic intervals for 24 weeks. These included the Brief Psychiatric Rating Scale (BPRS), Neuropsychiatric Inventory (NPI), Unified Parkinson's Disease Rating Scale (UPDRS) and tests of intellectual functioning, attention, and memory. Repeated measures statistical analysis was used to assess change from baseline. The results revealed significant improvements in the 24-week BPRS total score and NPI psychosis subscale scores, with no decline in UPDRS total or motor subscale scores. There was also significant improvement in recall scores on cognitive measures. These results indicate that quetiapine may treat psychotic symptoms and improve cognition without worsening motor function in patients with PD, suggesting that quetiapine is an effective and well-tolerated antipsychotic in this population.