To evaluate the relative utility of the Social Security Administration and National Death Index as sources of mortality data, the vital status of 12,866 participants in the Multiple Risk Factor Intervention Trial was identified from these sources and compared to the known mortality experience. The SSA correctly identified 87.8 per cent of the 409 deaths that occurred between 1974 and 1980. Underreporting of deaths by the SSA occurred for participants with certain demographic characteristics, especially marital status. For the years 1979 and 1980, the period for which the SSA and NDI have comparable data, the SSA correctly identified 93.2 per cent and the NDI correctly identified 98.4 per cent of the 191 known deaths. The NDI matching process resulted in a large number of false positive matches.