Study of Memantine to Treat Huntington's Disease

Inclusion Criteria: * Men or women aged 18 or older. * Diagnosis of HD with current complaints of memory or concentration difficulties. * Dementia Rating Scale score of \<129, to ensure that patients have sufficient cognitive impairment. * Adequate visual and auditory acuity to allow neuropsychological testing. * Good general health with no additional diseases expected to interfere with the study. * Patient is not institutionalized. * Sufficient English skills to complete all testing without assistance of an English language interpreter. * Availability of a responsible caregiver who agrees to supervise administration of study drug, monitor the patient's compliance and adverse events, and accompany the patient to all clinic visits. Exclusion Criteria: * 1\. Any significant neurologic disease other than HD. * Severe psychotic features or other severe psychiatric problems within the last three months which could lead to difficulty complying with the protocol. * History of alcohol or substance abuse within the past two years (DSM IV criteria). * Any significant systemic illness or unstable medical condition which could lead to difficulty complying with the protocol. * History of MI in the past year or head trauma with loss of consciousness greater than 20 minutes. * Insulin-requiring diabetes. * Use of any FDA approved cognitive enhancing prescription medications or investigational drugs within 30 days. * Use of ginkgo biloba or DHEA within four weeks prior to baseline. * Use of narcotic analgesics within 4 weeks prior to baseline. * Patients who, in the investigator's opinion, would not comply with study procedures.