The ratio of serum eicosapentaenoic acid (EPA) to arachidonic acid (AA) is significantly associated with long-term clinical outcomes in patients with acute myocardial infarction (AMI). However, it has not been conclusively demonstrated that higher serum EPA/AA ratio fares better clinical outcomes in the early phase of AMI. The Japanese registry of acute Myocardial INfarction diagnosed by Universal dEfiniTion (J-MINUET) is a prospective multicenter registry conducted in 28 Japanese medical institutions between July 2012 and March 2014. We enrolled 3,283 consecutive AMI patients who were admitted to participating institutions within 48 h of symptom onset. A serum EPA/AA ratio was available for 629 of these patients. The endpoints were in-hospital mortality and major adverse cardiac events (MACE), defined as a composite of all cause death, cardiac failure, ventricular tachycardia (VT) and/or ventricular fibrillation (VF) and bleeding during hospitalization. Although similar rates of in-hospital mortality, cardiac failure, bleeding, and MACE were found in the lower serum EPA/AA group and higher serum EPA/AA group, the incidence of VT/VF during hospitalization was significantly higher in the low ratio group (p = 0.008). Receiver operating characteristic curve analysis showed that an EPA/AA ratio < 0.35 could predict the incidence of VT/VF with 100% sensitivity and 64.0% specificity. A lower serum EPA/AA ratio was associated with a higher frequency of fatal arrhythmic events in the early phase of AMI.