This study was to assess the potential therapeutic efficacy of adipose-derived stem cells (ASCs) on infarcted hearts. Myocardial infarction was induced in rat hearts by occlusion of the left anterior descending (LAD) artery. One week after LAD occlusion, the rats were divided to three groups and subjected to transplantation of ASCs, transplantation of cell-culture medium (CCM) or remained untreated. During one-month recovery period, magnetic resonance (MR) imaging showed that the ASC-treated hearts had a significantly greater left ventricular ejection fraction (LVEF) and LV wall thickening than did the CCM-treated and untreated hearts. The capillary density in infarct border zone was significantly higher in the ASC-treated hearts than in the CCM-treated and untreated hearts. However, only 0.5% of the ASCs recovered from the ASC-treated hearts were stained positive for cardiac-specific fibril proteins. It was also found that ASCs under a normal culture conditioon secreted three cardiac protective growth factors, vascular endothelial growth factor, hepatocyte growth factor, and insulin-like growth factor-1. Results of this study suggest that ASCs were able to improve cardiac function of infarcted rat hearts. Paracrine effect may be the mechanism underlying the improved cardiac function and increased capillary density.