Background: Majority of acute myocardial infarction (AMI) patients who underwent successful PCI and had preserved left ventricular ejection fraction (LVEF) revealed favorable clinical course and good LVEF on follow-up echocardiography. We sought to investigate the predictors of decrement of LVEF in these patients. Methods: We enrolled total of 1,688 AMI patients (59.5짹11.8 years, 75.7% males) who underwent successful PCI and had preserved LVEF during peri-procedural period from Korea Acute Myocardial Infarction Registry (KAMIR) between Nov. 2005 and Jan. 2008. The patients were confirmed by significant coronary artery stenosis on coronary angiography and TIMI grade III flow by percutaneous coronary intervention (PCI). The patients were categorized into two groups by the presence (group I) or absence (group II) of decrement of LVEF (<50%) on echocardiography during one-year follow-up. Major adverse cardiac events (MACEs) included cardiac death, non-cardiac death, re-MI, coronary artery bypass graft, and re-PCI. Results: Group I was 138 patients (8.2%). There was no difference in the proportion of ST-elevation MI between two groups (69.3% vs. 62.4%, p=0.105). Baseline clinical characterstics had no differences except dyslipidemia (19.7% vs. 10.1%, p=0.001). LVEF during peri-procedural period was lower in group I (55.7짹5.9% vs. 60.6짹7.4%, p<0.001). Laboratory findings showed higher levels of CK (2120.4짹2026.7 vs. 1160.4짹1379.3 mg/dL, p<0.001), CK-MB (215.4짹182.6 vs. 159.5짹68.6 mg/dL, p<0.001), and troponin-I (71.5짹96.4 ng/mL vs. 36.7짹63.0 mL/min, p<0.001) in group I. Coronary angiographic findings showed more frequent involvement of LAD as a culprit lesion (49.3% vs. 40.3%, p=0.039) and lower pre-TIMI flow grade (p=0.005) in group I. The composite of MACE during follow-up period (1, 6, and 12 months) had higher incidence in group I (p=0.017, p=0.004, p<0.001, respectively). On multi-variate analysis, low LVEF (HR: Hazard Ratio 0.92, 95% CI: Confidence Interval 0.88-0.96, p<0.001), dyslipidemia (HR 2.87, 95% CI 1.65-5.0, p<0.001), LAD as a culprit lesion (HR 1.73, 95% CI 1.10-2.70, p=0.017), and high level of CK (HR 1.55, 95% CI 1.10-2.36, p=0.002) were the independent predictors of decrement of LVEF on follow-up period. Conclusion: Most AMI patients with preserved LVEF on presentation showed favorable clinical course. However, populations with decreased LVEF on follow-up examination had poorer clinical outcomes in spite of successful revascularization. Independent predictors of LVEF decrement during follow-up period were low LVEF, dyslipidemia, LAD lesion, and high level of CK.