In contrast to conventional assumption that myocytes are never renewed after birth, a growing body of evidence suggests that human cardiac myocytes might divide in myocardial infarction and severe heart failure. Bone marrow cells may also contribute to myocyte regeneration, when injected or mobilized into systemic circulation by cytokines. A clinical study demonstrated that intracoronary administration of autologous bone marrow significantly improved the cardiac function after acute myocardial infarction. No adverse effect was found. Cell therapy using adult stem cells is anticipated to be an effective treatment of heart failure.