Background: Granulocyte-colony stimulating factor(G-CSF) is one of the most promising stem cell mobilizer for treatment of ischemic heart disease. Stem cell mobilizing effects of G-CSF administration were not evaluated in patients with myocardial infarction. We evaluated stem cell mobilizing effects of G-CSF in patients with acute and old myocardial infarction. Methods and results: Nine patients(60 짹 10years old, 7men) underwent apheresis after 4days administration of G-CSF with dose of 10ug/kg/day. Five patients had acute myocardial infarction(AMI: 4 짹 2days after AMI) and four patients had old myocardial infarction(OMI: 94 짹 117days after AMI). FACS analysis was performed with apheresis products. Apheresis products from patients with age under 65(n=6) showed higher proportions of AC133-positive (0.7 짹 0.6 vs. 0%, p=0.04), CD34-positive (13.2 짹 11.2 vs. 2.6 짹 1.5%, p=0.17) and KDR-positive cells(15.4 짹 13.6 vs. 2.1 짹 0.5%, p=0.17). Apheresis yield from both age group were not different(1.4 짹 0.7 x 10⁹ vs. 1.3 짹 0.3 x 10⁹ from 1L of filtered blood, p=1.0). Although limited statistical significance, apheresis products from patients with AMI showed higher numbers of KDR-positive cell(3.4 짹 4.0 x 10⁸ vs. 0.7 짹 1.3 x 10⁸ from 1L of filtered blood, p=0.11), CD34-positive (2.5 짹 0.3 x 10⁸ vs. 0.5 짹 0.5 x 10⁸ from 1L of filtered blood, p=0.28), and AC133-positive cells(1.6 짹 2.6 x 10⁷ vs. 0.1 짹 0.3 x 10⁷ from 1L of filtered blood, p=0.55). Conclusions: Although limited numbers of study population, this results showed that stem cell mobilizing efficacy of G-CSF was influenced by age and presence of acute ischemia in patients with myocardial infarction. Patients with AMI and younger age had augmented responses to G-CSF induced stem cell mobilization. These data suggested that higher doses and longer administration schedule of G-CSF should be considered for patients with OMI and older age.