Marine n-3 polyunsaturated fatty acids may reduce coronary mortality. Previous data in relation to nonfatal coronary disease, however, have been inconsistent, which may be explained by the use of heterogeneous methods to assess the intake of marine n-3 polyunsaturated fatty acids. We investigated the hypothesis that the content of total and individual marine n-3 polyunsaturated fatty acids in adipose tissue is negatively associated with the incidence of acute coronary syndrome (ACS), including both fatal and nonfatal coronary disease. In the Diet, Cancer and Health, a Danish cohort study, 57 053 subjects were enrolled and had an adipose tissue biopsy taken at inclusion. During a mean follow-up period of 7.6 years, we identified and verified all cases (n=1012) with an incident acute coronary syndrome diagnosis, and a random sample of the cohort (n=1630) had their fatty acid composition in adipose tissue determined by gas chromatography. We found negative dose-response associations between the content of total marine n-3 polyunsaturated fatty acids and individual n-3 polyunsaturated fatty acids in adipose tissue and the risk of acute coronary syndrome. Comparing men in the highest and lowest quintiles gave a hazard ratio of 0.65 (95% confidence interval, 0.45 to 0.95) for total n-3 polyunsaturated fatty acids and 0.51 (95% confidence interval, 0.36 to 0.73) for docosahexaenoic acid. Nonfatal cases constituted >86% of cases, and the association was driven primarily by a reduction in the risk of nonfatal acute coronary syndrome. No consistent associations were found among women. Intake of marine n-3 polyunsaturated fatty acids may protect against acute coronary syndrome in men.