During the past few years, studies involving the implantation of stem cells, chemical factors, and scaffolds have demonstrated the ability to augment the mammalian heart's native regenerative capacity. Scaffolds comprised of extracellular matrix (ECM) have been used to repair myocardial defects. These scaffolds become populated with myocytes and provide regional contractile function, but quantification of the myocyte population has not yet been conducted. The purpose of this study was to quantitate the myocyte content within the ECM bioscaffold and to correlate this cell population with the regional mechanical function over time. Xenogenic ECM scaffolds derived from porcine urinary bladder were implanted into a full-thickness, surgically induced, right ventricular-free wall defect in a dog model. Zero, 2, and 8 weeks following implantation, regional function and myocyte content were determined in each patch region. Regional function did not significantly increase from 0 to 2 weeks. At 8 weeks, however, regional stroke work increased to 3.7 +/- 0.7% and systolic contraction increased to 4.4 +/- 1.2%. The myocyte content also significantly increased during that period generating a linear relationship between regional function and myocyte content. In conclusion, ECM used as a myocardial patch increases both the regional function and the myocyte content over time. The mechanical function generated in the patch region is correlated with the quantity of local tissue myocytes.