Medicine and epidemiology currently dominate the study of the strong association between socioeconomic status and mortality. Socioeconomic status typically is viewed as a causally irrelevant "confounding variable" or as a less critical variable marking only the beginning of a causal chain in which intervening risk factors are given prominence. Yet the association between socioeconomic status and mortality has persisted despite radical changes in the diseases and risk factors that are presumed to explain it. This suggests that the effect of socioeconomic status on mortality essentially cannot be understood by reductive explanations that focus on current mechanisms. Accordingly, Link and Phelan (1995) proposed that socioeconomic status is a "fundamental cause" of mortality disparities-that socioeconomic disparities endure despite changing mechanisms because socioeconomic status embodies an array of resources, such as money, knowledge, prestige, power, and beneficial social connections, that protect health no matter what mechanisms are relevant at any given time. We identified a situation in which resources should be less helpful in prolonging life, and derived the following prediction from the theory: For less preventable causes of death (for which we know little about prevention or treatment), socioeconomic status will be less strongly associated with mortality than for more preventable causes. We tested this hypothesis with the National Longitudinal Mortality Study, which followed Current Population Survey respondents (N = 370,930) for mortality for nine years. Our hypothesis was supported, lending support to the theory of fundamental causes and more generally to the importance of a sociological approach to the study of socioeconomic disparities in mortality.