The purpose of this study is to establish the efficacy of 20 mg/day and 40 mg/day doses of istradefylline for reducing the mean total hours of awake time per day spent in the OFF state in patients with advanced Parkinson's disease (PD) treated with levodopa. Patients who meet entry criteria will be randomized in a 1:1:1 ratio to double blind treatment with oral doses of 20 or 40 mg/day istradefylline or matching placebo. Patients will be treated for 12 weeks and will have interim visits and end of treatment visit to assess the efficacy and safety of istradefylline.