BACKGROUND: Low molecular weight heparin (LMWH) comparing with unfractionted heparin (UFH) in ST-elevation acute myocardial infarction (STEMI) patients receiving fibrinolytic therapy was known to be less inferior, but in patients with STEMI undergoing primary percutaneous coronary intervention (PCI), the benefit of LMWH on outcomes was not elucidate. OBJECTIVES: To compare the outcomes of patients treated with LMWH vs UFH and to define the role of LMWH in patients with STEMI at high risk for ischemic cardiac complications managed with primary PCI. METHODS: Patients undergoing primary PCI in the Korean Acute Myocriac Infarction Registry (KAMIR) were divided into two groups according to use of LMWH (n=692) or UFH (n=930) between October 2005 and July 2007. The primary end point was the all-cause death or myocardial infarction or major bleeding during the first 30 days after registration. RESULTS: The primary end point occurred in 4.7% (31/651) of patients managed with LMWH and 7.4% (59/799) of patients managed with UFH [odd ratio (OR), 0.64; 95% confidence interval (Cl), 0.415 – 0.991; p=0.045]. TIMI (Thrombolysis in Myocardial Infarction) major bleeding was observed only with UFH {0% vs 0.15% , p=NS), because none of patients were managed with thrombolysis. CONCLUSIONS: For patients of STEMI managed with a primary PCI, LMWH was not less beneficial than UFH. LMWH is a safe and effective alternative to UFH with the advantages of convenience.