IMPORTANCE: Despite its documented undercapture of mortality data, the US Social Security Administration Death Master File (SSDMF) is still often used to provide mortality end points in retrospective clinical studies. Changes in death data reporting to SSDMF in 2011 may have further affected the reliability of mortality end points, with varying consequences over time and by state. OBJECTIVE: To evaluate the reliability of mortality rates in the SSDMF in a cohort of patients with atherosclerotic cardiovascular disease (ASCVD). DESIGN, SETTING, AND PARTICIPANTS: This observational analysis used the IBM MarketScan Medicare and commercial insurance databases linked to mortality information from the SSDMF. Adults with ASCVD who had a clinical encounter between January 1, 2012, and December 31, 2013, at least 2 years of follow-up, and from states with 1000 or more eligible adults with ASCVD were included in the study. Data analysis was conducted between April 18 and May 21, 2018. MAIN OUTCOMES AND MEASURES: Kaplan-Meier analyses were conducted to estimate state-level mortality rates for adults with ASCVD, stratified by database (commercial or Medicare). Constant hazards of mortality by state were tested, and individual state Kaplan-Meier curves for temporal changes were evaluated. For states in which the hazard of death was constant over time, mortality rates for adults with ASCVD were compared with state-level, age group-specific overall mortality rates in 2012, as reported by the National Center for Health Statistics (NCHS). RESULTS: This study of mortality data of 667 516 adults with ASCVD included 274 005 adults in the commercial insurance database cohort (171 959 male [62.8%] and median [interquartile range (IQR)] age of 58 [52-62] years) and 393 511 in the Medicare database cohort (245 366 male [62.4%] and median [IQR] age of 76 [70-83] years). Of the 41 states included, 11 states (26.8%) in the commercial cohort and 18 states (43.9%) in the Medicare cohort had a change in the hazard of death after 2012. Among states with constant hazard, state-level mortality rates using the SSDMF ranged widely, from 0.06 to 1.30 per 100 person-years (commercial cohort) and from 0.83 to 6.07 per 100 person-years (Medicare cohort). Variability between states in mortality estimates for adults with ASCVD using SSDMF data greatly exceeded variability in overall mortality from the NCHS. No correlation was found between NCHS mortality estimates and those from the SSDMF (ρ = 0.29 [P = .06] for age 55-64 years; ρ = 0.18 [P = .27] for age 65-74 years). CONCLUSIONS AND RELEVANCE: The SSDMF appeared to markedly underestimate mortality rates, with variable undercapture among states and over time; this finding suggests that SSDMF data are not reliable and should not be used alone by researchers to estimate mortality rates.