Four groups of 20 subjects/group of subjects will be randomly assigned to receive CoQ10 (at doses of 300, 600 or 1200 mg/day) or matching placebo. A subject must have early PD that does not require treatment. A subject will be evaluated at a Screening Visit, a Baseline Visit (after which the subject begins to take the study drug) and visits at 1, 4, 8, 12, and 16 months after the baseline visit. The subject will be followed until she/he needs treatment with levodopa or for a maximum of 16 months. Because CoQ10 is a component of the mitochondria (the part of a cell that makes energy), blood samples will be taken at the baseline visit and the last visit to study mitochondrial function.