OBJECTIVE: To identify the emerging clinical precursors that indicate the early onset of Huntington disease (HD) in a reliable and gene-specific manner. This information is critical for the development of therapeutic trials aimed at postponing clinical onset in HD gene carriers. METHODS: Between July 1999 and January 2004, 1001 adults at 50-50 risk for HD agreed to provide longitudinal clinical data and a blood DNA sample under consent provisions that require their individual clinical and genetic information to never be revealed. RESULTS: The Prospective Huntington At Risk Observational Study (PHAROS) cohort is characterized by a 2:1 predominance of women to men, high educational attainment, and gainful employment. Despite the gender disparity, the demographic, hereditary, and clinical characteristics of the female and male participants were similar. Investigators, who are unaware of individual gene status, characterized the baseline cohort to be highly functional with minimal motor or cognitive impairment; 92.3% of participants were judged to have no or nonspecific motor abnormalities; 6.7%, to have possible or probable motor signs; and only 1.0%, to have unequivocal HD. CONCLUSION: The baseline characteristics of the PHAROS cohort make it well suited to generate objective and prospective data about gene-specific clinical precursors that can be used as outcomes in controlled trials aimed at postponing the onset of HD.