Background: The efficacy of intracoronary infusion of peripheral blood stem cells needs to be evaluated in patients with acute (AMI) versus old myocardial infarction (OMI) in the setting of drug eluting stent (DES) implantation in randomized clinical trial. Methods and Results: Among 96 patients enrolled in this study, 89 patients with myocardial infarction who underwent coronary revascularization with DES for the culprit lesion completed 6 months follow-up; AMI-cell infusion (n=25), AMI-control (n=25), OMI-cell infusion (n=19), and OMI-control group (n=20). Among enrolled patients, 5 patients were dropped out during follow up (3 patients in AMI-control group and 2 patients in AMI-cell infusion group) and 2 patients were expired during follow up (cardiac death in 1 patient from AMI-control group and noncardiac death in 1 patient from OMI-cell infusion group). Cell infusion groups received intracoronary infusion of collected PBSCs, mobilized by G-CSF for 3 days. The AMI-cell infusion group showed a significant additive improvement in left ventricular ejection fraction (LVEF) and remodeling compared with controls (change of LVEF: 52.0짹9.9% to 57.1+8.7% vs. 53.2짹13.3 to 53.1짹11.5%, p<0.05, change of end-systolic volume: 69.0짹21.5mL to 63.5짹20.3mL vs 62.7짹30.9mL to 69.2짹38.0mL, p<0.05). In OMI patients, however, there was no significant change of LVEF and ventricular remodeling in spite of significant improvement of coronary flow reserve following cell infusion. Although improvement of LVEF was not evident in OMI, cell infusion significantly improved coronary flow reserve and reduced infarct volume both in OMI and AMI. G-CSF-based cell therapy did not aggravate neointimal growth with DES implantation (p>0.05). Conclusion: Intracoronary infusion of mobilized PBSCs with G-CSF improves LVEF and remodeling in patients with AMI, but less definite in patients with OMI. G-CSF based stem cell therapy with DES implantation is both feasible and safe, eliminating any potential for restenosis.