There is a paucity of data on outcomes in patients undergoing percutaneous coronary intervention for ST segment elevation myocardial infarction (STEMI) caused by left main stem (LMS) occlusion. We sought to identify the clinical features and outcomes of patients with STEMI due to LMS culprit lesions as compared with those due to non-LMS culprit lesions. From 4,720 consecutive patients with STEMI who underwent PCI in the Korean Acute Myocardial Infarction Registry from November 2005 to January 2008, we selected 61 patients with unprotected LMS lesions and 4636 with non-LMS. Clinical outcomes were major adverse cardiac events (MACE) 1, 6 and 12 months after PCI. The LMS group had a higher incidence of total MACE (26.2% vs. 7.83%, P<0.001) and cardiac death (23.0% vs. 4.4%, P<0.001) at 12 months. However, the mortality after 1 month in the LMS group was similar to that in the non-LMS group.The changes of hemodynamic features were important of the cardiac death in the LMS group. Predictors of Cardiac death in LMS group were shock (HR 5.98 Cl 1.36 to 26.31 P<0.018), and heart rate (HR 1.050 Cl 1.01 to 1.09 P<0.006). In conclusion, these data suggest that a high mortality in the STEMI of LM lesions may be due to hemodynamic deterioration. However, long-term MACE after 1 month may not be different between STEMI due to LMS and non-LMS .