Dietary intake of marine n-3 PUFA has been negatively associated with the risk of CHD among subjects with known CHD, whereas an effect in healthy subjects is less documented. We assessed the hypothesis that dietary intake of marine n-3 PUFA is negatively associated with the risk of acute coronary syndrome (ACS) in healthy subjects. In the Danish Diet, Cancer and Health cohort study, 57 053 participants were enrolled. Dietary intake of total n-3 PUFA, including EPA, docosapentaenoic acid (DPA) and DHA, was assessed. During a mean follow-up period of 7·6 years, we identified all cases (n 1150) from this cohort with an incident ACS diagnosis in the Danish National Patient Registry or the Cause of Death Registry. Diagnoses were verified through medical record review. In Cox proportional hazard models, we adjusted for established risk factors for CHD. Men in the four highest quintiles of n-3 PUFA intake (>0·39 g n-3 PUFA per d) had a lower incidence of ACS compared with men in the lowest quintile. The hazard ratio was 0·83 (95 % CI 0·67, 1·03) when we compared men in the second lowest and lowest quintile of n-3 PUFA intake. Higher intake of n-3 PUFA did not strengthen this association. Associations for EPA, DPA and DHA were all negative, but less consistent. No convincing associations were found among women. In conclusion, we found borderline significant negative associations between the intake of marine n-3 PUFA and ACS among healthy men.