Objective: Adult mesenchymal stem cells (MSCs) have the potential candidate for cell replacement therapy to treat many myocardial diseases. It is thought that adult human MSCs do not induce immunoreactivity even to xenografts. We investigated the effects of these human MSCs in rat myocardial infarction model. Methods: Bone marrow was aspirated from healthy human. We isolated and grew MSCs via previously proven method by PHARMICELL Inc. These MSCs were about 5 passages cell with MSC surface markers (positive 5 markers; SH2, SH3, CD 29, CD 44, CD 106, negative 2 markers; CD 14, CD 45). An infarction was created by left anterior descending (LAD) artery ligation of 25 Sprague-Dawley rats (BW: 294 g). We are divided 2 groups; control vs. MSCs group. After LAD artery ligation, 1 x 10⁶ MSCs or saline were multiple injected around the infarcted area using a 30-gauze tuberculin syringe (1cc volume). All the rats were sacrificed at day 28 and histological analysis by hematoxylin-eosin, x-gal and immunohistochemical staining. We investigated functional improvement of infarcted myoicardium via transthoracic echocardiography (VIVID 7, GE medical, 12MHz probe). Results: Myocardial infarction caused approximately 28% mortality (7 / 25 rats). There were not significantly difference in mortality between the two groups (control; 4/13, MSCs; 3/12 rats, p>0.05). Immunfluorescence studies revealed some engrafted cells expressing a smmoth muscle phenotype (alpha actin) or cardiomyocyte (troponin I) and stained x-gal in MSCs group. After 28 days, echocardiography showed an improvement on left ventricular performance in the MSCs compared with the control group (fractional shortening; 22.1% vs. 39.2%, p=0.032). Conclusion: In the setting of rat myocardial infarction, the transplantation of human MSCs improves left ventricular function.