The Unified Huntington's Disease Rating Scale (UHDRS) adequately measures decline in patients at early and moderate stages of Huntington's disease (HD). In advanced patients, floor effects hamper the evaluation, thus calling for an adjusted scale. We designed the UHDRS-For Advanced Patients (UHDRS-FAP), in order to improve longitudinal assessment of patients at advanced disease stage. Sixty-nine patients with a Total Functional Capacity (TFC) ≤ 5 were recruited in France and in the Netherlands. Among them, 45 patients were followed longitudinally (mean 1.6 ± 1.2 years) with the UHDRS-FAP; 30 were also assessed with the UHDRS. Cross-sectional analyses evaluated psychometric properties and interrater reliability of the scale. Longitudinal analyses evaluated the sensitivity to decline compared to the UHDRS. Internal consistency was higher for motor and cognitive scores than for somatic and behavioral scores (0.84, 0.91, 0.70, and 0.49, respectively). Interrater reliability was ≥ 0.88 in all scores. The somatic score, specific to the UHDRS-FAP, declined over time, as well as motor and cognitive performance with both scales. Although performance with the 2 scales correlated, the UHDRS-FAP appeared more sensitive to change and was the only scale that detected decline in patients with a TFC ≤ 1. Neither scale detected a significant decline in behavioral scores. The UHDRS-FAP is reliable and more sensitive to change than the original UHDRS for cognitive and motor domains. It offers items relevant for daily care. Behavioral scores tended to decline but this may reflect the decline in the communicative abilities of the patients. © 2013 International Parkinson and Movement Disorder Society.