The MITIGATE-HD study is a single center Phase IIb,out-patient,randomized, double-blind, placebo-controlled trial of memantine treatment in subjects with Huntington disease (HD). The study will evaluate Memantine 10 mg two times daily (BID) administered orally (PO) for six months (24 weeks) compared with matching placebo BID. Safety and tolerability will be assessed by recording of adverse events and by monitoring of vital signs, physical examinations, and suicidality risk scores.