A reduced risk of fatal coronary artery disease has been associated with a high intake of (n-3) fatty acids (FA) and a direct cardioprotective effect by their incorporation into myocardial cells has been suggested. Based on these observations, the omega-3 index (eicosapentaenoic acid + docosahexaenoic acid in cell membranes of RBC expressed as percent of total FA) has been suggested as a new risk marker for cardiac death. In this study, our aim was to evaluate the omega-3 index as a prognostic risk marker following hospitalization with an acute coronary syndrome (ACS). The omega-3 index was measured at admission in 460 patients with an ACS as defined by Troponin-T (TnT) > or = 0.02 microg/L. During a 2-y follow-up, recurrent myocardial infarctions (MI) (defined as TnT > 0.05 microg/L with a typical MI presentation) and cardiac and all-cause mortality were registered. Cox regression analyses were used to relate the risk of new events to the quartiles of the omega-3 index at inclusion. After correction for age, sex, previous heart disease, hypertension, diabetes, smoking, high-sensitivity C-reactive protein, brain natriuretic peptide, creatinine, total cholesterol, HDL-cholesterol, triacylglycerol, homocysteine, BMI, and medication, there was no significant reduction in risk for all-cause mortality, cardiac death, or MI with increasing values of the index. In conclusion, we could not confirm the omega-3 index as a useful prognostic risk marker following an ACS.