Background: Metabolic syndrome (MS) is highly prevalent among patients with acute myocardial infarction (MI) and associated with increased risk for cardiovascular diseases and related mortalities. The aim of this study was to investigate the effects of MS on the clinical outcomes after primary percutaneous coronary intervention (PCI) in acute ST elevation myocardial infarction (STEMI) patients. Methods: By employing data from Korea Acute MI Registry (KAMIR; from November 2005 to December 2007), a total of 3,736 patients suffered from acute STEMI who underwent primary PCI were categorized according to the NCEP ATP III criteria of MS. Primary study outcomes included major adverse cardiac events (MACE: cardiac death, non-cardiac death, re-MI, re-PCI, and coronary artery bypass surgery) during one-year follow-up. Results: Group I with MS (n=2,292, 1,574 men, 60.9짹12.7 years) and group II without MS (n=1,444, 1,235 men, 61.7짹12.7 years) were categorized. Group I had higher rate of multi-vessel involvement and in-hospital death compared with group II (p<0.001, p<0.001, respectively). During one-year follow-up, the rates of composite MACE at 1, 6, and 12 months did not show any differences between two groups. Multivariate analysis showed that old age (���65 years), low HDL-cholesterol level (male <40 mg/dL, female <50 mg/dL) and multi-vessel involvement were the independent predictors of in-hospital mortality. Conclusions: Although MS did not influence the long-term clinical outcomes after primary PCI in patients with acute STEMI, it was associated with higher rate of multi-vessel involvement and in-hospital death.