This study assessed safety of transendocardial (TE) electromechanical-guided delivery of bone marrow mesenchymal stem cells (MSCs) after acute myocardial infarction (AMI) and compared intracoronary (IC) delivery with TE delivery. In a canine acute myocardial ischemia model, 100 x 10(6) MSCs were delivered 7 days after AMI via IC and TE routes. Functional assessment was performed by 2D echocardiogram, and detailed histopathologic analyses were performed to assess the impact of cell therapy in vascular density and fibrosis. Patterns of cell distribution in both delivery methods were also compared. There was a statistically significant reduction in the amount of myocardial ischemia in the TE group (P=0.007). Left ventricular ejection fraction (LVEF) increased 13% (mean) in the TE group (21-day follow-up) and was significantly better than that of the controls (P=0.01), but did not improve in the IC-delivery group. Dissimilar patterns of cell distribution were noted between the IC and TE groups. This study suggests that MSC treatment is probably safe and effective after AMI. In the comparison of TE and IC delivery, the TE group showed higher cell retention (clusters even in the injury center of the infarct) with an increased vascularity and greater functional improvement than did the IC group (no clusters; cells at the border of the infarct). The higher local cell density in the TE group may be important for therapeutic effectiveness.