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.

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 40mg/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.

Inclusion Criteria:

1. UK Parkinson's Disease Society (UKPDS) brain bank criteria (Step 1 and 2) for PD.
2. PD stages 2-4 in the OFF state for Modified Hoehn and Yahr Scale.
3. On levodopa/dopa-decarboxylase inhibitor for at least one year.
4. Taking at least three doses and >=300mg of levodopa per day for at least four weeks before randomization.
5. Predictable end of dose wearing off.
6. Able to satisfactorily complete Hauser based 24-hour patient Parkinson's diary.
7. Have an average of two hours of OFF time on 24-hour diaries.
8. On a stable regimen of any other anti-Parkinson's drugs for at least four weeks before randomization.
9. Be at least 20 years of age.
10. Be willing and able to give written informed consent.

Exclusion Criteria:

1. Taking any excluded medications.
2. Neurosurgical treatment or Transcranial Magnetic Stimulation for PD.
3. Diagnosis of cancer within 5 years.
4. Diagnosis of clinically significant illness of any organ system.
5. Diagnosis of dementia or mini-mental status examination score of 25 or less.
6. History of drug or alcohol abuse or dependence within the past two years.
7. History of psychosis.
8. Significant drug allergies.
9. Taking anticonvulsants for seizures.
10. History of neurological malignant syndrome.
11. Pregnant or lactating females.