We performed a 6-month open-label trial to evaluate the tolerability and efficacy of coenzyme Q10 (CoQ) in 10 patients with Huntington's disease (HD), Subjects were evaluated at baseline, 3 months, and 6 months using the HD Rating Scale (HDRS), the HD Functional Capacity Scale (HDFCS), and standardized neuropsychological measures. Adverse events (AEs) were assessed by telephone interview every month. CoQ doses ranged from 600 to 1,200 mg per day. All subjects completed the study, although four subjects reported mild AEs, including headache, heartburn, fatigue, and increased involuntary movements. There was no singnificant effect of the treatment on the clinical ratings. The good tolerability of CoQ suggests that it is a good candidate for evaluation in liong–term clinical trials designed to slow the progression of HD.