To assess the utility of the National Death Index (NDI) in improving the ascertainment of deaths among people diagnosed with acquired immunodeficiency syndrome (AIDS), the authors determined the number and characteristics of additional deaths identified through NDI linkage not ascertained by using standard electronic linkage with Florida Vital Records and the Social Security Administration's Death Master File. Records of people diagnosed with acquired immunodeficiency syndrome between 1993 and 2007 in Florida were linked to the NDI. The demographic characteristics and reported human immunodeficiency virus (HIV) transmission modes of people whose deaths were identified by using the NDI were compared with those whose deaths were ascertained by standard linkage methods. Of the 15,094 submitted records, 719 had confirmed matches, comprising 2.1% of known deaths (n = 34,504) within the cohort. Hispanics, males, people 40 years of age or older, and injection drug users were overrepresented among deaths ascertained only by the NDI. In-state deaths comprised 59.0% of newly identified deaths, and human immunodeficiency virus was less likely to be a cause of death among newly identified compared with previously identified deaths. The newly identified deaths were not previously ascertained principally because of slight differences in personal identifying information and could have been identified through improved linkages with Florida Vital Records.