SummaryA relationship between serum polyunsaturated fatty acids (PUFAs) and cardiovascular disease has been reported; however, the existence of a relationship between serum PUFAs and extent of vessel disease (VD) in patients with ST elevation myocardial infarction (STEMI) remains unclear.Between July 2011 and June 2015, 866 consecutive STEMI patients underwent emergent percutaneous coronary intervention, 507 of whom were enrolled and classified into three groups according to the initial angiograms: 1VD, 294 patients; 2VD, 110 patients; and 3VD/left main trunk disease (LMTD), 103 patients. Serum levels of PUFAs, including eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and arachidonic acid, and other laboratory data during hospitalization were evaluated.The serum EPA level in the 3VD/LMTD group was significantly lower than that in the 1VD group (55.5 ± 22.1 versus 66.2 ± 28.7, P = 0.002) and was slightly lower than that in the 2VD group (55.5 ± 22.1 versus 65.2 ± 28.9, P = 0.0167). Multivariate adjustment analysis revealed that age !70 years (odds ratio, 1.72; 95% confidence interval, 1.03-2.89; P = 0.038) and a low serum EPA level (odds ratio, 0.98; 95% confidence interval, 0.99-1.00; P = 0.023) were independent risk factors for 3VD/LMTD, while a low serum DHA level was not.A low serum EPA level may be more strongly related than a low serum DHA level to the extent of VD in STEMI patients. Age !70 years and a low serum EPA level may be independent risk factors for 3VD/LMTD.(Int Heart J 2017; 58: 868-873) Key words: Eicosapentaenoic acid, Docosahexaenoic acid, Mulivessel disease, Acute coronary syndrome, Cardiovascular disease A n association between fish intake and risk of cardiovascular disease has been demonstrated in several epidemiological studies and clinical trials.