Abstract: AIM: To study the optimal time and safety of intramyocardial, intracoronary or intravenous (i.v.) transplantation of autologous bone-marrow-derived mononuclear cells (BM-MNCs) into ischemic myocardium of acute myocardial infarction (AMI) in swine. METHODS: Transplantation was performed immediately after myocardial infarction in intramyocardial group and i.v. group, whereas transplantation was performed 1 week after myocardial infarction in intracoronary group. Four weeks later, blood vessel density, heart function, collateral circulation formation and side effects were observed and compared. RESULTS: Blood vessel density in the three BM-MNCs transplantation groups was significantly higher than in their respective control group (P<0.01). No significant difference was observed in blood vessel density between intramyocardial group and intracoronary group but blood vessel density in intramyocardial group was significantly higher than in i.v. group (P<0.01). No significant difference was found in left ventricular ejection fraction (LVEF) and fraction shortening of short axis (FS) among the three transplantation groups. Left ventricular end-diastolic pressure (LVDEP) and left ventricular end-diastolic dimension (LVEDd) in intramyocardial group and intracoronary group were lower than in control group, but no significance was observed between i.v. group and control group. One week after transplantation, the levels of basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) in the three transplantation group were significantly higher than those in control group and before operation (P<0.01). No side effects such as abnormal angiogenesis, calcification or cancer were observed in the three transplantation groups. CONCLUSIONS: No obvious side effects are observed in intramyocardial, intracoronary or i.v. transplantation of BM-MNCs into myocardium of AMI in swine. The optimal time for intramyocardial or i.v. transplantation is immediately after myocardial infarction and the optimal time for intracoronary transplantation is 1 week after myocardial infarction. Intramyocardial, intracoronary or i.v. transplantation all induces angiogenesis in the ischemic myocardium and improves left ventricular systolic function. The therapeutic efficacy of intramyocardial or intracoronary transplantation is similar but better than the efficacy of i.v. transplantation. Intramyocardial and intracoronary transplantation improves left ventricular diastolic function and attenuates remodeling but these effects are not as effective in i.v. transplantation.