The study objective is to determine the efficacy of Coenzyme Q10 (CpQ) in Huntington's disease (HD). Although the genetic defect that causes HD has been identified there is no known effective treatment or cure. Rational therapeutic strategies in Huntington's disease include those that are targeted to improving cellular energy production and reducing oxidative stress. Coenzyme Q10, a co-factor involved in mitochondria! electron transfer and an anti-oxidant, is a compound that has some of these properties. Coenzyme Q10 slows progression and prolongs survival in a dose dependent manner in a transgenic mouse model of HD. Coenzyme Q10 at a dosage of 600 mg per day for 2 years appeared to slowed the functional decline in HD by approximately 13% compared to placebo. Pre-clinical and clinical studies with COQ suggest that higher dosages are more beneficial. The study hypothesis is that chronic treatment of HD patients with CoQ will slow the progressive functional decline of HD. The specific aim is to test this hypothesis by conducting a double-blind, placebo-controlled, randomized, parallel group, multi-center study of CoQ involving 608 ambulatory HD subjects who are each treated for 60 months. Eligible subjects will be randomized to CoQ 2400 mg daily or matching placebo. A CoQ dosage of 2400 mg was chosen based on dosage ranging tolerability studies in HD, amyotrophic lateral sclerosis and Parkinson's disease. The primary outcome measure will be the clinical progression of HD as measured by the change in the total functional capacity (TFC) between baseline and 60 months. Secondary measures will include changes in the other clinical rating scales of the Unified Huntington Disease Rating Scale (UHDRS), times to decline in TFC by 2 and 3 points, ability to complete the study at the assigned dosage and the frequencies of clinical and laboratory adverse events.