In the present study, we tested the hypothesis that granulocyte colony-stimulating factor (G-CSF) administration would enhance the efficacy of cellular cardiomyoplasty with embryonic stem (ES) cells in infarcted myocardium. Three weeks after myocardial infarction by cryoinjury, Sprague-Dawley rats were randomized to receive either an injection of medium, ES cell transplantation, G-CSF administration, or a combination of G-CSF administration and ES cell transplantation. Prior to transplantation, ES cells were induced to differentiate into cardiomyocytes. Eight weeks after treatment, the cardiac tissue formation, neovascularization, and apoptotic activity in the infarct regions were evaluated by histology and immunohistochemistry. The left ventricular (LV) dimensions and function of the treated heart were evaluated by echocardiography. Transplanted ES cells survived and participated in the myocardial regeneration in the infarcted heart. A combination of G-CSF treatment and ES cell transplantation significantly promoted angiogenesis and reduced the infarct area and cell apoptosis in the infarcted myocardium compared with ES cell transplantation alone. The combination therapy also attenuated LV dilation, ascompared with ES cell transplantation alone. The present study demonstrates that G-CSF treatment can enhance the efficacy of cellular cardiomyoplasty by ES cell transplantation to treat myocardial infarction.