We compared vital status follow-up by the National Death Index and the Social Security Administration for a cohort of breast cancer patients. Only the National Death Index allowed follow-up for subjects with an unknown Social Security number. All of the deaths identified by the Social Security Administration were reported by the National Death Index. No subject reported to be alive by the Social Security Administration matched a National Death Index record. Subjects with inaccurate identifying information were more effectively followed up by the National Death Index. The National Death Index more accurately reported dates of death.