The death of brain cells in Huntington Disease (HD) is thought to be associated with a lack of normal cell energy and harmful brain substances called free radicals. Coenzyme Q10 (CoQ) is a marketed nutritional supplement that may prove useful in HD because it increases cell energy and combats free radicals.

Most studies of CoQ have looked at only one formulation of CoQ ("ubiquinone") in HD. The purpose of the study is to find out if people that switch from the common formulation of CoQ ("ubiquinone") to a different formulation ("ubiquinol") have higher levels of CoQ in their blood after taking the same dose. The investigators also want to find out if this different formulation is tolerable for individuals with HD.

Inclusion Criteria:

• Have manifest Huntington disease
• Be 18 years of age or older
• Be taking an oxidized formulation of CoQ for at least 30 days prior to the baseline visit
• Be on a steady dose of all concomitant medications for at least 30 days prior to the baseline visit

Exclusion Criteria:

• Have a history of intolerability of sensitivity to CoQ
• Have an unstable medical or psychiatric illness
• Be pregnant or breastfeeding; women of childbearing age must use reliable contraception