This paper discusses the current data concerning the results of major clinical trials using bone marrow-derived and peripheral blood-derived stem/progenitor cells in treatment of patients with acute myocardial infarction (AMI) and depressed left ventricular ejection fraction. In all major trials (TOPCARE-AMI, BOOST), the primary outcome measure was increase in left ventricular systolic function (LVEF) and left ventricle remodeling. The most consistent finding is the significant increase in LVEF. Some trials suggest also reduction of left ventricular remodeling. Although the absolute LVEF increase is small (6-9%), it may substantially contribute to the improvement of global LV contractility. None of the studies in AMI patients treated with intracoronary infusion of progenitor cells revealed excess risk of arrythmia, restenosis or other adverse effects attributable to the therapy. The exact mechanism of improved myocardial contractile function remains unknown, however, there are several possible explanations: therapeutic angiogenesis improving the blood supply to the infarct border zone, paracrine modulation of myocardial fibrosis and remodeling (e.g. inhibition of myocyte apoptosis) and transdifferentiation of stem/progenitor cells into functional cardiomyocytes. No study showed the superiority of the particular subpopulation of autologous progenitor cells in terms of left ventricular function improvement in AMI. In fact, most of the clinical trials used the whole population of mononuclear bone marrow-derived progenitor cells, peripheral blood derived progenitor cells (endothelial progenitors).