This study is a multicenter Phase 3, randomized, double-blind, placebo-controlled safety and efficacy study of Dimebon treatment in subjects with Huntington disease (HD). The study will evaluate Dimebon 20 mg three times daily (TID) administered orally (PO) for six months (26 weeks) compared with matching placebo TID for the primary safety and efficacy analyses. Safety and tolerability will be assessed by recording of adverse events and by monitoring of vital signs, physical examinations, safety laboratory evaluations, and 12-lead electrocardiogram(ECG)assessments.