The primary purpose of this study is to evaluate the efficacy, safety, and tolerability of 40 mg per day of istradefylline (KW6002) as monotherapy in patients with Parkinson's disease.

Parkinson's disease is a progressive disease which results in deterioration of motor function and is the result of dopamine depletion in specific brain structures. Current therapeutic approaches include dopamine replacement and the use of dopamine receptor agonist drugs. These therapies are effective but may be associated with unwanted complications like wearing off phenomena and involuntary abnormal movements (dyskinesia). Istradefylline may provide a nondopaminergic approach to the treatment of Parkinson's disease.

This study will compare the efficacy of 40 mg per day of istradefylline in improving the symptoms of Parkinson's disease with placebo.

Inclusion Criteria:

1. Early PD by UKPDS criteria
2. Mild to moderate difficulty daily activities
3. Females: Either postmenopausal or willing to use adequate contraception

Exclusion Criteria:

1. Unable to discontinue current PD medication
2. Exposure to Levodopa for more than 1 month
3. Symptoms that may suggest a diagnosis other than Parkinson's disease
4. Medical conditions and/or abnormal laboratory findings which preclude participation including cancer in the last 5 years, a history of drug abuse/dependence, abnormal cognitive status, a history of seizures, neuroleptic malignant syndrome, psychosis, or abnormal liver function tests