AscertaInment of the vital status of individuals is of central importance to epidemiologic studies which monitor mortality as an end point. Utilizing Identifying Information collected In 1973–1974, the Hypertension Detection and Follow-up Program, a prospective, multiceriter study, followed 25,362 indMduals to deter mIne eight-year mortality. In the most recent follow-up, there were 617 individuals whose vital status was not known. Available Identifying information on these and on all 1, participants known to have died in 1979–1981 was submitted to the National Death Index (NDI) for possible confirmation of vital status. A subset of individuals who had Social Security numbers (490 lost to follow-up and 1,154 known deaths) was also submitted to the Social Security Administration (SSA). The NDI correctly identified 87.0% of the known deaths. Of the 1,154 known deaths (those with known Social Security numbers) submitted to both agencies, the NDI identified 93.1% and the SSA 83.6%. Significant variations by race and sex were noted in the identification rates, in part because of Social Security number discrepancies. False matches through the NDI matchIng process oc curred for 10.4% of the known deaths. In the more restrictive SSA search, only 0.5% false matches resulted. For those lost to follow-up, vital status was ascer tained In 57.1%. This paper describes the relative efficacy and attributes of the use of these systems to ascertain vital status.