Eleven patients with Parkinson's disease (PD) and acute psychosis received flexible doses of quetiapine between 25 and 300 mg/day based on clinical response and tolerance. Ten patients were receiving dopaminergic agents at baseline. Serial efficacy ratings (Brief Psychiatric Rating Scale, Clinical Global Impressions Scale), neuromuscular symptom assessments (Abnormal Involuntary Movement Scale, Simpson-Angus Scale, Unified Parkinson's Disease Rating Scale [UPDRS]), and adverse events monitoring were performed for up to 52 weeks. The patients had moderate hallucinations and/or delusions at baseline before the initiation of quetiapine. Nine of the 11 patients completed at least 12 weeks of treatment. Quetiapine was well tolerated in all but one patient, who became dizzy within the first week and withdrew from the study. Ten patients presented with moderate visual hallucinations. Quetiapine was markedly effective in controlling visual hallucinations in six of these patients. Symptoms of paranoia or delusions were less responsive to quetiapine. Four patients withdrew because of adverse events or comorbid medical problems, two withdrew because of a lack of efficacy, and five completed 52 weeks of treatment. The introduction of quetiapine did not exacerbate parkinsonian symptoms. Motor dysfunction, as measured by the UPDRS, revealed a slow, gradual worsening consistent with the progression of PD. Atypical antipsychotic medications such as quetiapine have a reduced likelihood of causing adverse drug-induced parkinsonism and therefore a possible role in treating psychotic symptoms in patients with PD.