The purpose of this research study is to see if tetrabenazine, which is commonly used to treat Huntington's Disease (HD), reduces the problems of impulsivity that are common in patients with HD. Investigators will also see how the medicine affects aspects of thinking and mood.

This study is an open-label assessment of behavioral symptoms including depression, impulsivity, and suicidal ideations in patients prior to, and after, taking a stable dose of TBZ for Huntington's disease. All subjects will be evaluated with tests of depression (Beck Depression Scale), impulsivity (QUIP, BIS-11, and computerized impulsivity scales), and suicidal ideation (Columbia Suicide Scale) as a safety measure before they actually start taking TBZ. Since there is a significant lack of awareness of emotional and cognitive symptoms (anosagnosia) in patients with HD, collateral sources will be asked to fill out similar questionnaires based on their knowledge and observation of the patient. Patients and collaterals will be asked to return to the clinic for an identical evaluation plus adverse events, after they have been on a stable dose of TBZ for 4 weeks, which would be 8 ± 1 week after initiating TBZ. Patients will be titrated over 3-5 weeks to best dose as determined by the investigator, up to a maximum of 75 mg/day. As patients are titrating, there will be several telephone contacts conducted. The initial dose will be 12.5 mg BID. Best dose will be largely determined by adverse events and subjective efficacy. Determination of best dose will include two scheduled phone calls and others as needed. Other medications will remain stable between visits.

Inclusion Criteria:

• For HUntington's Disease (HD) patients only--Definite HD as indicated by positive gene testing or typical symptoms in the context of family history of HD.
• A moderate degree of impulsivity as measured by the Barrat Impulsivity Scale (BIS). (>65)
• Must be symptomatic in the opinion of the investigator. Standard clinical criteria for symptomatic HD will be employed, any motor signs c/w HD, usually chorea.
• Patient is cognitively alert and able to answer/understand.

Exclusion Criteria:

• Patient requires the assistance of another person to walk, or is non-ambulatory.
• Patient is severely impaired cognitively.
• Patients taking neuroleptic (dopamine blocking) medications within the past 14 days.
• patient is actively suicidal, has untreated or inadequately treated depression, has impaired hepatic function, is taking MAO inhibitors or is taking reserpine or has been off of reserpine for less than 20 days