Background: In this prospective nonrandomized open-labeled study, we investigated whether the intracoronary infusion of autologous peripheral blood stem cells (PBSC) in patients with successfully reperfused acute myocardial infarction could safely improve myocardial function. Methods: We enrolled only patients with left anterior descending artery occlusion were enrolled to minimize bias related to the coronary artery anatomy. Ten patients who underwent successful percutaneous coronary intervention (PCI) 8.3짹8.1 days after AMI and agreed to clinical trial received granulocyte-colony stimulating factor for 4 days. Then PBSC (2.0짹0.7x10^9 mononuclear cells including 7.9짹5.7X10^6 CD34+ cells) were selectively infused into culprit coronary artery using transport catheter. To maximize myocardial localization of PBSC, proximal site of vessel was occluded by complaint balloon during cell infusion. Results: After 6 month follow-up, left ventricular function evaluated by echocardiography and magnetic resonance imaging (MRI), and microvascular function evaluated by Doppler wire were improved in PBSC treatment group (left ventricular ejection fraction (LVEF); 37.2짹7.5 vs 47.4짹13.5% , p=0.027; coronary flow reserve; 1.90짹0.50 vs 2.84짹0.79, p=0.004). There was no in-stent restenosis or major adverse clinical event. However, comparable improvement in left ventricular function was also shown in control groups who underwent elective PCI 8.4짹7.0 days after AMI (N=31; LVEF; 41.9짹8.0 vs 47.7짹11.9%; p<0.001) or primary PCI (N=32; LVEF; 42.6짹6.3 vs 48.6짹7.1%; p<0.001). Conclusion: Our preliminary result shows that for patients with AMI and undergone PCI, additional intracoronary infusion of PBSC after AMI is safe and feasible, but may not lead to significant difference of improvement in myocardial function compared to standard PCI strategy.