Cell therapies have emerged as a promising treatment for the prevention of heart failure after myocardial infarction (MI). This study evaluated the capacity of an aligned, fibrin-based, stretch-conditioned cardiac patch consisting of either the native population or a cardiomyocyte (CM)-depleted population (i.e., CM+ or CM- patches) of neonatal rat heart cells to ameliorate left ventricular (LV) remodeling in the acute-phase postinfarction in syngeneic, immunocompetent rats. Patches were exposed to 7 days of static culture and 7 days of cyclic stretching prior to implantation. Within 1 week of implantation, both patches became vascularized, and non-CMs began migrating from CM+ patches. By week 4, patches had been remodeled into collagenous tissue, and live, elongated, donor CMs were found within grafted CM+ patches. Significant improvement in cardiac contractile function was seen with the administration of the CM+ patch (ejection fraction increased from 35.1% ± 4.0% for MI only to 58.8% ± 7.3% with a CM+ patch, p<0.05) associated with a 77% reduction in infarct size (61.3% ± 7.9% for MI only, 13.9% ± 10.8% for CM+ patch, p<0.05), and the elimination of LV free-wall thinning. Decreased infarct size and reduced wall thinning also occurred with the administration of the CM- patch (infarct size 36.9% ± 10.2%, LV wall thickness: 1058.2 ± 135.4 μm for CM- patch, 661.3 ± 37.4 μm for MI only, p<0.05), but without improvements in cardiac function. Approximately 36.5% of the transplanted CMs survived at 4 weeks; however, they remained separated and electrically uncoupled from the host myocardium by a layer of CM-free tissue, which suggests that the benefits of CM+ patch transplantation resulted from paracrine mechanisms originating from CMs. Collectively, these observations suggest that the transplantation of CM-containing engineered heart tissue patches can lead to dramatic improvements in cardiac function and remodeling after acute MI.