BACKGROUND: The prognostic importance of noninfarct-related left main coronary artery disease (LMD; ���30% stenosis) in the setting of acute myocardial infarction (AMI) is unknown. METHODS: Of 7,673 consecutive patients with AMI who had undergone PCI from November 2005 to January 2008 and were enrolled in Korean Acute Myocardial Infarction Registry, 1.3% (n=99) had noninfarct-related LMD. Major adverse cardiac events (MACE) such as cardiac death, nonfatal MI and repeat revascularization were compared between AMI patients with and without noninfarct-related LMD at 1 month and 12 months after PCI. RESULTS: Of 99 AMI patients with noninfarct-related LMD, 40 patients (44.4%) with left main stenosis more than 50% were treated with PCI, while patients with 30-50% left main stenosis were conservatively managed. They were older, had higher body mass index and more ischemic heart disease history than those without LMD (p<0.01, respectively). Patients with LMD had significantly higher MACE rates at 1 month (7.1% vs. 3.0%, p=0.021) and 12 months (19.2% vs. 7.7%, p=0.001). Patients with LMD had significantly higher cardiac death rates (7.1% vs. 2.2%, p=0.001) and repeat revascularization rates (10.1% vs. 4.4%, p=0.007) at 12 months. After risk adjustment for differences in baseline profiles, noninfarct-related LMD were independent predictor associated with 1-year MACE. (hazard ratio 1.773, 95% confident interval 1.072 to 2.933, p=0.026). CONCLUSION: AMI patients with noninfarct-related LMD have worse short-term and long-term prognosis. Though not culprit, coronary artery disease of left main itself may be an important prognostic indicator in the setting of AMI.