Background: There are several studies about prognosis of silent myocardial ischemia but few data are available regarding painless ST-segment elevation myocardial infarction (STEMI), a much more severe form of ischemia. The aim of this study was to determine the incidence, clinical characteristics, and outcomes of painless STEMI. Methods: We analyzed the large-scale, prospective, observational KAMIR (Korea Acute Myocardial Infarction Registry) study, which enrolled 7,288 STEMI patients (61.8 짹 12.8 years, 74.3% men) that presented with or without chest pain [Painless STEMI group (n=763) and Painful STEMI group (n=6,525)]. Patients with cardiogenic shock were excluded in the analysis. Painless STEMI was defined as STEMI without pain symptoms and dyspnea, which is an accepted angina equivalent. End points were in-hospital mortality and 1-year major adverse cardiac events (MACE). Results: Painless STEMI included 10.5% of total STEMI patients. Patients with painless STEMI were older (65.2 짹 12.8 vs. 61.4 짹 12.8, p<0.001) and more likely to be female (30.4% vs. 25.1%, p=0.001), non-smokers (57.6% vs. 62.5%, p=0.009), diabetic (29.0% vs. 24.2%, p=0.003), normo-lipidemic (33.2% vs. 39.1%, p=0.002), and had higher Killip class (Killip III: 12.0% vs. 7.2%, p<0.001). The painless group had more multi-vessel coronary disease (59.0% vs. 52.8%, p=0.003) but less pre-TIMI flow 0 (45.2% vs. 53.2%, p<0.001) and complex lesions (71.2% vs. 78.4%, p<0.001). Drug-eluting stents were used less frequently in the painless group (86.2% vs. 91.4%, p<0.001) and physicians used paclitaxel-eluting stents more to the patients in the painless group than to those in the painful group (43.2% vs. 30.5%, p<0.001). Regarding clinical outcomes, the painless group had more in-hospital death (5.9% vs. 3.6%, p = 0.026) and 1-year MACE (26.0% vs. 19.1%, p = 0.002). In Cox proportional hazards analysis, hypotension (HR 4.40, 95% CI 1.41 to 13.78, p = 0.011), low left ventricular ejection fraction (HR 3.12, 95% CI 1.21 to 8.07, p = 0.019), and high Killip class (HR 3.48, 95% CI 1.19 to 10.22, p = 0.023) were independent predictors of 1-year MACE in patients with painless STEMI. Conclusion: Painless STEMI was associated with more adverse outcomes than painful STEMI and late detection may have contributed significantly to total ischemic burden. This results warrant more investigations for methodological development in diagnosis of silent ischemia and painless STEMI.