Background and objectives: Many studies demonstrated that metabolic syndrome (MS) is associated with increased risk for cardiovascular diseases and related mortalities. We sought to determine the prevalence of MS in patients with acute myocardial infarction (AMI) and its effect on clinical outcomes. Methods: Employing data from the Korea Acute Myocardial Infarction Registry (KAMIR; November 2005 to December 2006), a total of 1,990 patients suffered from acute ST-elevation myocardial infarction (STEMI) were categorized according to the NCEP ATP ��� criteria of MS. Primary study outcomes included major adverse cardiac events (MACE: cardiac death, non-cardiac death, re-MI, repeat percutaneous coronary intervention and coronary artery bypass surgery) during one-year follow-up. Results: Patients were grouped based on existence of MS: group ���: MS (n=1,182, 777 men, 62.8짹12.3 years of age); group ���: Non-MS (n=808, 675 men, 64.2짹13.1 years of age). Group ��� showed lower left ventricular ejection fraction (LVEF) (p=0.005). There were no difference between two groups in the coronary angiographic findings except for multivessel involvement (p=0.01). The incidence of in-hospital death was significantly higher in group ��� than in group ��� (p=0.047), but during follow-up, the rates of composite MACE at 1, 6 and 12 months showed no significant differences between the two groups. Multivariate analysis showed that low LVEF, old age, low HDL-cholesterol level and multivessel involvement are associated with high in-hospital death rate. Conclusions: MS in patients with STEMI was an important predictor for in-hospital death, which suggested the need for early identification and medical intervention for secondary prevention of MS.