Background: The superiority of low-molecular-weight heparin (LMWH) compared with conventional unfractionated heparin (UFH) in patients with non-ST-elevation myocardial infarction (NSTEMI) undergoing early invasive management is controversey. We compared the clinical outcomes and safety profiles of LWMH and UFH in these patients. Methods: Between October 2005 and January 2008, we enrolled a total of 1,550 NSTEMI patients undergoing early invasive management in KAMIR registry. Each patient received LMWH [and/or adjunctive UFH during percutaneous coronary intervention (PCI), n=640] or UFH [n=910] as antithrombotic agent. Major and minor bleeding, mortality in hospital and death or myocardial infarction (MI) at 30 days were assessed. Results: Triple antiplatelet medications (aspirin, thienopyridine, and cilostazol) were more prescribed in UFH group (34.7% vs. 22.7%, p<0.001). There was no difference in use of glycoprotein IIb/IIIa inhibitor before or during PCI (14.0% vs. 13.8%, p=0.941). Major and minor bleeding complications were 0.1% in UFH group and 0.3% in LMWH group (p=0.573). In-hospital mortality was similar between both groups (2.2% vs. 1.6%, p=0.455). In clinical outcomes at 30 days, there were trend to lower death/MI in LMWH group (4.3% vs. 2.4%, p=0.076). On subgroup analysis, LMWH reduced 30-day all-cause death/MI in low risk patients [TIMI (Thrombolysis In Myocardial Infarction) risk score <4]. Conclusion: The LMWH improved short-term clinical outcomes in relatively low risk patients with NSTEMI undergoing early invasive management without differences in major bleeding compared with UFH. KEYWORDS: Low-molecular-weight heparin; Unfractionated heparin; non-ST-elevation myocardial infarction