Age at onset of motor symptoms was collected on 611 persons affected with Huntington disease (HD) among 3,201 persons "at risk" in 108 kindreds. Life-table estimates correcting for truncated intervals of observation (censoring) produced a median age at onset 5 years older than the observed mean. Risk estimates of HD onset for persons at risk, as calculated by life-table methods, were significantly higher for older ages than were estimates based on the observed distribution of onsets. Age-specific incidence was found to be highest at age 35-64 years, a considerably older age interval than suggested by previous estimates. The offspring of affected males had significantly younger onset than did offspring of affected females, and a trend suggesting and excess of paternal descent among juvenile-onset cases was present. Life-table analysis is contrasted with analyses of (a) the observed distribution of age at onset and (b) remote cohorts age 63 or older at the time of data collection. The implications for risk prediction, genetic counseling, and genetic analysis of HD are discussed.