Coenzyme Q10 (CoQ10), a potential neuroprotective compound, was previously investigated at a dosage of 600 mg/day in Huntington's disease (HD) patients and demonstrated a trend toward slowing disease progression. Higher CoQ10 dosages may prove beneficial. We investigated the tolerability and blood levels associated with 1,200, 2,400, and 3,600 mg/day of CoQ10 in HD and healthy subjects. Twenty-eight subjects (20 HD, 8 healthy) enrolled in a 20-week open-label trial. Subjects started on 1,200 mg/day of CoQ10, increasing every 4 weeks by 1,200 mg to a maximum dosage of 3,600 mg/day. Monthly evaluations included review of adverse events and CoQ10 blood levels. Twenty-three subjects (82%) achieved the target dosage of 3,600 mg/day. Six subjects (2 healthy, 4 HD) withdrew prematurely (gastrointestinal (GI) symptoms in 3, worsening HD in 2, and 1 because of a fall). All three serious adverse events occurred in a single subject, and were deemed unrelated to CoQ10. The most common adverse events seen were GI symptoms. Mean (± SD) CoQ10 blood levels achieved over the course of the trial were as follows: 1.26 ± 1.27 μg/mL (baseline, n = 28), 5.59 ± 2.24 μg/mL (1,200 mg/day, week 4, n = 26), 6.38 ± 3.25 μg/mL (2,400 mg/day, week 8, n = 25), 7.49 ± 4.09 μg/mL (3,600 mg/day, week 12, n = 23), and 6.78 ± 3.36μg/mL (3,600 mg/day, week 20, n = 20). CoQ10 was well tolerated with over 80% of subjects achieving the target dosage. Dosages of 2,400 mg/day may provide the best balance between tolerability and blood level achieved. Further studies examining the efficacy of 2,400 mg/day are planned. © 2010 Movement Disorder Society.