The Huntington's Disease Activities of Daily Living (HD-ADL) scale is a 17-item informant-completed instrument for rating adaptive functioning in Huntington's disease (HD) patients. To assess the reliability and clinical correlates of the HD-ADL, it was administered along with the Mini-Mental State Exam (MMSE) and Quantified Neurological Exam (QNE) to 93 HD patients. The HD-ADL has high internal consistency (coefficient alpha = 0.91). Principal components factor analysis of the HD-ADL revealed four factors (General Functioning, Domestic Activities, Home Upkeep, and Family Relationships) accounting for 72% of the total variance. MMSE score, motor impairment score, and duration of disease were the best predictors of HD-ADL score. Similar results were obtained in an independent replication sample of patients. The HD-ADL scores correlate with Shoulson and Fahn's total functional capacity (TFC) index (r = -0.89). The HD-ADL scale is a reliable and valid instrument for assessing adaptive function in HD patients.