Background : Acute myocardial infarction(AMI) caused by left main coronary artery(LMCA) as culprit lesion is rare but it carries a very high mortality rate. However, there have been few data on the incidence, predictors, clinical feature and outcomes in this clinical setting. The purpose of this study was to know the clinical characteristics and prognostic determinants in patients with AMI caused by LMCA lesion. Methods and Results : We performed analysis of patients with AMI who underwent emergency coronary angiography between January 2004 and December 2007. Out of 1,550 patients with AMI, 38 patients (2.5 %) had LMCA disease as the culprit lesion. Baseline characteristics of patients were mean age, 68짹11 years; male gender, 52.6 %; dyslipidemia 42.1%; diabetes 36.8%; hypertension 34.2% and smoking 27.8%. Fifteen patients (39.5%) were ST elevation myocardial infarction(STEMI) and seven patients(18.4%) were arrived in cardiogenic shock. Six patients (15.8%) underwent cardiopulmonary resuscitation. Twenty five patients (65.8%) underwent percutaneous coronary intervention and nine patients ( 23.7%) underwent coronary byass graft with comparable procedure success rate ( 76.0% vs 77.8%). In-hospital mortality rate was 28.9%. The two most prognostic determinant of hospital mortality were ST elevation on electrocardiography and low ejection fraction, At six month follow up, 69.2% of hospital survivals were taking triple antiplatelet therapy and mortality rate increased to 31.5%. Conclusions : Our clinical observation suggests patients with AMI with widespread precordial ST elevation or unusual ST change on electrocardiography complicated by catastrophic presentation should be suspected LMCA disease which show high mortality rate despite of early aggressive management. The two most prognostic determinant of hospital mortality were ST elevation on electrocardiography and low ejection fraction,