The primary objective of this study was to establish the long-term tolerability and safety of istradefylline treatment in subjects with Parkinson's disease treated with levodopa/carbidopa. In addition, treatment response and maintenance of response were assessed.

Inclusion Criteria:

Eligible subjects were:

• male or female
• at least 30 years of age
• had completed participation in a prior double-blind istradefylline trial
• met United Kingdom's Parkinson's Disease Society (UKPDS) brain bank criteria (Step 1 and Step 2) for Parkinson's disease and the severity of the Parkinson's disease was defined as Stages 2-4 on the Modified Hoehn and Yahr Scale while in the OFF state
• had been treated with levodopa for at least 1 year
• had been on a stable Parkinson's disease regimen within normal therapeutic ranges including levodopa for at least 4 weeks before Baseline
• were currently taking at least 4 doses of levodopa per day (3 doses per day if at least 2 doses contained slow-release formulation)
• had predictable end-of-dose wearing-off

Exclusion Criteria:

• none