Parkinson's Disease is a progressive neurodegenerative disorder characterized by bradykinesia, rigidity, tremor and abnormal posture and gait. Many patients can have mild to moderate symptoms, while others with advanced disease have symptoms which interfere with activities of daily living to a severe degree. Although effective in addressing motor dysfunction, long-term use of anti-Parkinsonian agents has been implicated as a component in the development of psychiatric side effects including psychosis. Treatment of psychosis with typical antipsychotics is not recommended in this patient population, since even low potency typical antipsychotics can cause marked exacerbations of parkinsonism in Parkinson's disease patients. The use of atypical antipsychotics (e.g., clozapine, risperidone and quetiapine) has shown some efficacy in the treatment of psychosis in PD patients. Melperone is classified atypical antipsychotic. European experience with melperone spans more than 30 years, and it encompasses an established antipsychotic efficacy profile in the treatment of confusion, anxiety, unrest (particularly in the elderly) and schizophrenia as well as a favorable safety and tolerability profile. Eligible subjects with Parkinson's disease psychosis will participate in a 1-2 week Screening/Washout Period, a 5 week Titration Phase (one of three doses of melperone or placebo), a 1 week Maintenance Phase and a Taper/Follow-up Period up to 2 weeks. Following the Day 43 assessment, subjects may be given the option of receiving melperone in an open-label extension study.