When the patient's optimal dose is determined treatment will begin. Patients will take tablets or capsules containing KW-6002 or placebo (a look-alike pill with no active ingredient) once a day for 2 weeks, in addition to their regular Sinemet. All participants will receive placebo at least 2 weeks during the study; some patients will receive only placebo throughout the entire 7 weeks. At the end of weeks 1, 3 and 5, patients will be evaluated with a brief physical examination, routine blood and urine tests, and assessment of any adverse effects.