OBJECTIVE: The aim of this study was to investigate the association between the content of marine n-3 polyunsaturated fatty acids (PUFA) in adipose tissue, a biomarker for the long-term intake of seafood, and the subsequent development of breast cancer (BC). DESIGN: We designed a case-cohort study based on a cohort of healthy Danish women, who in the 1990s donated adipose tissue biopsies to a biobank in order to investigate the role of diet for the development of cancer and chronic disease. During follow-up, incident cases of BC were identified through national registries, and the content of n-3 PUFA in adipose tissue was compared between cases and the cohort sample. RESULTS: During follow-up, 463 new cases of BC were identified. After adjusting for potential confounders, no significant association between the content of marine n-3 PUFA and BC was found. When comparing the highest with the lowest quintile, the hazard ratio (HR) was 0.96 (95% CI 0.64-1.43) for total marine n-3 PUFA, 0.84 (95% CI 0.58-1.23) for eicosapentaenoic acid (EPA), and 1.08 (95% CI 0.73-1.58) for docosahexaenoic acid (DHA). CONCLUSION: This study does not indicate any association between the content of total or individual marine n-3 PUFA in adipose tissue and development of BC.