Human mesenchymal stem cells (hMSCs) have only a limited differentiation potential toward cardiomyocytes. Forced expression of the cardiomyogenic transcription factor myocardin may stimulate hMSCs to acquire a cardiomyogenic phenotype, thereby improving their possible therapeutic potential. hMSCs were transduced with green fluorescent protein (GFP) and myocardin (hMSC(myoc)) or GFP and empty vector (hMSC). After coronary ligation in immune-compromised NOD/scid mice, hMSC(myoc) (n = 10), hMSC (n = 10), or medium only (n = 12) was injected into the infarct area. Sham-operated mice (n = 12) were used to determine baseline characteristics. Left ventricular (LV) volumes and ejection fraction (EF) were serially (days 2 and 14) assessed using 9.4-T magnetic resonance imaging. LV pressure-volume measurements were performed at day 15, followed by histological evaluation. At day 2, no differences in infarct size, LV volumes, or EF were observed among the myocardial infarction groups. At day 14, left ventricular ejection fraction in both cell-treated groups was preserved compared with the nontreated group; in addition, hMSC(myoc) injection also reduced LV volumes compared with medium injection (p < .05). Furthermore, pressure-volume measurements revealed a significantly better LV function after hMSC(myoc) injection compared with hMSC treatment. Immunohistochemistry at day 15 demonstrated that the engraftment rate was higher in the hMSC(myoc) group compared with the hMSC group (p < .05). Furthermore, these cells expressed a number of cardiomyocyte-specific markers not observed in the hMSC group. After myocardial infarction, injection of hMSC(myoc) improved LV function and limited LV remodeling, effects not observed after injection of hMSC. Furthermore, forced myocardin expression improved engraftment and induced a cardiomyocyte-like phenotype hMSC differentiation.