Emerging evidence indicates that cardiomyocytes can be regenerated from embryonic stem cells, adult mesenchymal stem cells, and cardiac side population cells (cardiac stem cells). When these cells were injected into diseased hearts, indices of cardiac function improved. There are ethical difficulties involved in the use of embryonic stem cells. The ratio of differentiation into cardiomyocytes is still relatively low at present. Transplantation of regenerated cardiomyocytes or stem cells, however, will become a future therapy for severe heart failure.