Background: In patients with acute ST-elevation myocardial infarction (STEMI) who were performed primary coronary angiography (CAG) sometimes observed to have already been reperfused even before coronary intervention (PCI). Spontaneous reperfusion (SR) is known to improve clinical outcome, yet its incidence is unclear. We tried systemic analysis of SR using Korean Acute Myocardial Infarction Resistry (KAMIR) database. Methods: 5995 patients were enrolled from KAMIR data who had STEMI underwent primary PCI. The SR incidence, predictive factors, relationship with major adverse cardiac event (MACE) were analysed. The SR was defined as to have TIMI III flow at the time of primary coronary angiography. Results: Overall, SR defined as pre-PCI TIMI III flow occurred in 15.1% of patients. 96.1% of patients with SR and 87.9% of patients with no-SR showed post-PCI TIMI III flow (p<0.001). Patients without SR had worse epicardial flow, higher baseline blood LDL-cholesterol level (no SR; 117.5짹 44.4, SR; 113.4짹 37.7, p=0.01) and tended toward less ST resolution post PCI than those with SR. The patients with DM rather showed good results (17.1% of SR) than non-DM (14.4% of SR). The patients with SR showed good hospital survival rate (96.9% vs 94.2%, p=0.002), lower MACE rate at 1 month (20.9% vs 26.0%, p=0.02), 6 months (37.2% vs 44.5%, p<0.001), and 12 months (51.7% vs 59.1%, p<0.001). Multivarate analysis showed that SR was associated with 6 months MACE (CI 0.63-0.86, OR 0.74, p<0.001). Conclusion: These results suggest that SR is associated with comparable subsequent epicardial flow post-PCI and improved clinical outcome.