BACKGROUND: There is a paucity of data on outcomes in patients undergoing percutaneous coronary intervention (PCI) for ST segment elevation myocardial infarction (STEMI) caused by left main stem (LMS) occlusion. We sought to identify the clinical features and outcomes of STEMI with LMS culprit lesion as compared with that of non-LMS culprit lesion. METHODS: From 4697 consecutive patients with STEMI who had undergone PCI from November 2005 to January 2008 in Korean Acute Myocardial Infarction Registry (KAMIR), 61 cases (1.3%) with unprotected LMS culprit lesion were identified. Primary outcome was major adverse cardiac events (MACE: cardiac death, non-fatal myocardial infarction and target vessel revascularization) at 1 month and 12 months after PCI. RESULTS: There was no difference in clinical history (hypertension, diabetes, age, gender, body mass index and smoking) except past history of ischemic heart disease (19.7% vs. 9.8%, p=0.01) between the two groups. But hemodynamic features presenting systolic blood pressure (108.7짹33.73 vs. 124.93짹29.89, p=0.001), heart rate (86.34짹22.53 vs. 75.48짹20.16, p=0.001) were worse in LMS group. Initial glucose level (216.69짹102.69 vs. 176.22짹78.79, p= 0.004) and LDL-cholesterol level (104.27짹37.97 vs. 116.94짹37.44, p=0.024) are different between the two groups. LMS group had higher rates of MACE at 1month (21.3% vs. 4.1%, p=0.001). LMS group had higher rates of MACE at 12 months (27.9% vs. 8.6%, p=0.001), higher rates of cardiac death (23.0% vs. 5.0%, p=0.001). But there is no difference rates of nonfatal MI (0% vs. 0.6%, p=0.535) and target vessel revascularization (4.9% vs. 3.1%, p=0.419) between the two groups. STEMI with LMS culprit (hazard ratio 2.411, 95% confident interval 1.298 to 4.478, p=0.005) were significantly associated with increased MACE at 12 months. CONCLUSION: LMS culprit STEMI had more past history of ischemic heart disease than non-LMS culprit STEMI. LMS culprit STEMI had more MACE at 1month and 12months than non-LMS culprit STEMI.