AIM: To investigate the effects of in situ transplantation of mobilized autologous bone marrow stem cells on infarction size and cardiac function in patients with acute myocardial infarction. METHODS:25 patients with first acute myocardial infarction were randomly divided into stem cells in situ transplantation group and control group, 12 patients in stem cells in situ transplantation group were injected subcutaneously with 300 ?g granulocyte colony-stimulating factor(G-CSF) daily for four days in addition to standard therapy. 13 patients in control group were treated with standard therapy alone. The conventional 12 leads electrocardiogram were recorded on 1, 28 days after admission and the cardiac function was scored by the QRS scoring system proposed by Wagner. Furthermore, the infarction size was assessed by radionuclide myocardial perfusion imaging 7, 28 days after admission. RESULTS:4 weeks after admission, the QRS scores decreased, the infarction size reduced significantly in the stem cells in situ transplantation group (from 36.0%?8.3% to 18.0%?5.8%, P