Background: The mobilization of hematopoietic progenitor cells from bone marrow has been proposed to play a role in cardiac regeneration after myocardial infarction(MI). Accordingly, an increase in CD34 positive cells(CD34+) has been observed in the peripheral blood of patients after acute myocardial infarction. Here, we evaluated the influence of an acute percutaneous coronary intervention(PCI) of the occluded artery on the mobilization of CD34+in acute MI. Methods: CD34 positive cells were quantified by flow cytometry(FACS analysis) and expressed as number per million white blood cells. Peripheral blood was obtained and analyzed at day 5 after the onset of symptoms from patients with acute MI without early PCI(n=11, age 63±5 years), acute MI with rapid PCI(n=7, age 63±3), patients with pneumonia(n=5, age 51±6), patients without angiographical signs of coronary artery disease(control, n=5, age 66±8) and young healthy volunteers(n=11, age 28±1). Results: Patients with MI but without PCI had a higher CD34+count at day 5(312±48 per 106 leukocytes) than control(156±40, P=0.03) and MI with PCI(173±31, P=0.03). No increase in CD34+was observed in patients who underwent PCI vs. control. Patients with pneumonia had higher CD34+(350±44) than patients with MI with PCI(P=0.01) and control(P=0.01). Healthy individuals who were much younger than all other groups(28±1 years, P< 0.0001 vs. all groups) had the highest CD34+(526±51, P=0.006 vs. MI without PCI, P=0.00003 vs. MI with PCI, P=0.02 vs. pneumonia, P=0.00006 vs. control). Conclusions: Shorter time of ischemia and reduced cell death may be the reasons for reduced CD34+cell count after acute MI with early percutaneous intervention vs. acute MI without intervention. Besides ischemia, also inflammation as present in pneumonia may cause a mobilization of CD34+cells. Age may be a major factor that influences the mobilization of CD34+cells and the regenerative capacity of the heart. Background: The mobilization of hematopoietic progenitor cells from bone marrow has been to play a role in cardiac regeneration after myocardial infarction (MI). myocardial infarction. Here, we evaluated the influence of an acute percutaneous coronary intervention (PCI) of the occluded artery on the mobilization of CD34 + in acute MI. Methods: CD34 positive cells were quantified by flow cytometry (FACS analysis) and expressed as number per million white blood cells were obtained and analyzed at day 5 after the onset of symptoms from patients with acute MI without early PCI (n = 11, age 63 ± 5 years), acute MI with rapid PCI (n = 7, age 63 ± 3), patients with pneumonia (n = 5, age 51 ± 6), patients without angiographical signs of coronary artery disease (control, n = 5, age 66 ± 8) and young healthy volunteers 28 ± 1). Results: Patients with MI but witho ut PCI had a higher CD34 + count at day 5 (312 ± 48 per 106 leukocytes) than control (156 ± 40, p = 0.03) and MI with PCI (173 ± 31, p = 0.03) observed in patients who underwent PCI vs. control. Patients with pneumonia had higher CD34 + (350 ± 44) than patients with MI with PCI (P = 0.01) and control (P = 0.01). Healthy individuals who were much younger than all other groups (28 ± 1 years, P <0.0001 vs. all groups) had the highest CD34 + (526 ± 51, P = 0.006 vs. MI without PCI, P = 0.00003 vs. MI with PCI, P = 0.02 vs. pneumonia, P = 0.00006 vs. control). Conclusions: Shorter time of ischemia and reduced cell death may be the reasons for reduced CD34 + cell count after acute MI with early percutaneous intervention vs. acute MI without intervention. cause a mobilization of CD34 + cells. Age may be a major factor that influences the mobilization of CD34 + cells and the regenerative capacity of the heart.