The authors compared the ability of the National Death Index and the Equifax Nationwide Death Search to ascertain deaths of participants in the Nurses' Health Study. Each service was sent information on 197 participants aged 60-68 years in 1989 whose deaths were reported by kin or postal authorities and 1,997 participants of the same age who were known to be alive. Neither service was aware of the authors' information regarding participants' vital status. The sensitivity of the National Death Index was 98 percent and that of Equifax was 79 percent. Sensitivity was similar for women aged 65-68 years; however, for women aged 61-64 years, the sensitivity of the National Death Index was 97.7 percent compared with 60.2 percent for Equifax. The specificity of both services was approximately 100 percent. The contrast between the sources of these databases and the matching algorithms they employ has implications for researchers and for those planning health data systems.