Introduced several decades ago, the dogma persists that cardiac myocytes are terminally differentiated cells and that division of muscle cells is impossible in the adult heart. More recently, nuclear mitotic divisions in myocytes occasionally were seen, but those observations were challenged on the assumption that the rate of cell proliferation was inconsequential for actual tissue regeneration. Moreover, mitoses were never detected in normal myocardium. However, the analysis of routine histologic preparations constituted the basis for the belief that myocytes were unable to reenter the cell cycle and divide, ignoring the limitations of these techniques. We report here by confocal microscopy that 14 myocytes per million were in mitosis in control human hearts. A nearly 10-fold increase in this parameter was measured in end-stage ischemic heart disease (152 myocytes per million) and in idiopathic dilated cardiomyopathy (131 myocytes per million). Because the left ventricle contains 5.8 × 10⁹ myocytes, these mitotic indices imply that 81.2 × 10³, 882 × 10³, and 760 × 10³ myocytes were in mitosis in the entire ventricular myocardium of control hearts and hearts affected by ischemic and idiopathic dilated cardiomyopathy, respectively. Additionally, mitosis lasts less than 1 hr, suggesting that large numbers of myocytes can be formed in the nonpathologic and pathologic heart with time. Evidence of cytokinesis in myocytes was obtained, providing unequivocal proof of myocyte proliferation.