This is a Phase 3, 52-week, open-label, flexible-dose, multinational, multicenter study to evaluate the safety and tolerability of istradefylline 20 or 40 mg/d in subjects with moderate to severe PD with motor fluctuations and dyskinesia on levodopa combination (levodopa/carbidopa or levodopa/benserazide) therapy plus at least one adjunctive PD medication. Subjects who completed 12 weeks of double-blind treatment and the 30-day follow-up period in Study No. 6002-014 will undergo Screening and Baseline evaluations for eligibility for the study. Eligible subjects will be treated with istradefylline at a starting dose of 20 mg/d with an option for a dose adjustment to 40 mg/d at Week 12 based on the Investigator's judgment of each subject's response and tolerability. If deemed necessary, one unscheduled dose adjustment visit between Week 2 to Week 12 is allowed in accordance with clinical judgment of the Investigator. Subjects who had a dose adjustment to 40 mg/d can have their dose decreased to 20 mg/d by the Investigator at a second unscheduled dose adjustment visit if there are tolerability issues. The istradefylline dose should remain fixed between Week 26 to Week 52. Consultation with the Sponsor's Medical Monitor is required prior to any unscheduled dose adjustment visits. A subject may discontinue from the study at any time.

Inclusion Criteria:

1. Written informed consent;
2. Subjects who completed Study No. 6002-014 inclusive of the 30-day follow-up period;
3. Currently taking levodopa combination (carbidopa/levodopa or benserazide/levodopa) therapy plus at least one adjunctive PD medication;
4. Women of child-bearing potential (WOCBP) must use a reliable method of contraception and have a negative serum pregnancy test at Screening;

Exclusion Criteria:

1. Subjects with less than 70% treatment compliance throughout their enrollment on Study No 6002-014 and or with major protocol deviations in Study No. 6002-014 (subjects who failed to meet any of the inclusion criteria, subjects who met any of the exclusion criteria or subjects who met the criteria for subject withdrawal but who were not withdrawn);
2. Subjects on apomorphine and/or dopamine receptor antagonists or direct gastrointestinal levodopa infusion;
3. Subject who have had neurosurgical operation for PD;
4. Subjects taking A2a antagonist, potent CYP3A4 inhibitors, potent CYP34A inducers;
5. Subjects who have had a diagnosis of cancer or evidence of continued malignancy within the past three years (with the exception of adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer, or in situ prostate cancer with normal prostate-specific antigens post resection).