Background: In patients with acute ST-segment elevation myocardial infarction (STEMI) who were performed primary percutaneous coronary intervention (PCI) sometimes observed to have already been reperfused even before PCI. Spontaneous reperfusion (SR) is known to improve clinical outcome, yet its incidence and clinical importance is unclear. We tried systemic analysis of SR using Korean Acute Myocardial Infarction Resistry (KAMIR) database. Methods: 2,660 consecutive patients who had STEMI on primary ECG at the first medical contact time were divided into two groups according to the presence of SR; presence SR (group І; n=232 , 60.4짹12.2 years) vs no SR (group ����; n=2,428 , 60.9짹12.8 years). The SR incidence, survival rate, relationship with each of major adverse cardiac event (MACE) were analysed. The SR was defined as to have ST elevation at the first medical contact time followed by self resolution of ST elevation. Results: Overall, SR occurred in 8.8% of patients. The patients with SR did not show any difference in hospital survival rate (94.8% vs 95.0%, p=0.875), MACE rate at 1 month (1.6% vs 2.5%, p=0.619), 6 months (5.9% vs 9.4%, p=0.184), and 12 months (9.2% vs 14.6%, p=0.130). Each MACE events - cardiac or non-cardiac death, CABG, re-PCI, and MI - also did not have any statistical differences between SR and no-SR patients. On lab data, only maximum CK (1021.31짹 1298 vs 2061.20짹 2383, p<0.001) and maximum Troponin I(32.87짹 55 vs 70.30짹 108, p<0.001) showed difference. Diabetes and hypertension neither had any relationship with SR. Conclusion: These results suggest that SR defined by ECG change has little relationship with MACE and survival rate. Although SR has known to have some relationships with better clinical outcome, this study was not able to find any visible difference between SR and no-SR patients.