Both experimental and human clinical studies executed in the last 5 years suggested that bone marrow derived cells may participate in the healing process after myocardial infarction. A number of small clinical trials indicated mild or moderate beneficial effect of intracoronary administration of bone marrow derived stem cells after myocardial infarction. Most of the studies used mononuclear cell fraction; due to the cellular heterogeneity of this cell population the type of the effective subpopulation was not known. We investigated the safety and functional effects of the autologous bone marrow CD34+ stem cells after intracoronary administration in patients with recent myocardial infarction. 8 patients with impaired left ventricular function were transplanted with CD34+ bone marrow stem cells 12 +/- 1 day after the acute coronary event. 2D-echocardiography, FDG-PET and MIBI-SPECT were performed before transplantation and 6 month later. During the 6-month follow-up the global left ventricular function (basal EF 37.3 +/- 2.9%, after cell therapy 44.8 +/- 4.1%) and regional viability / metabolism increased significantly (17.6 +/- 13.5%). The increase of myocardial perfusion in the infarct region was tendentious but not significant. Our results demonstrate for the first time that the CD34+ subpopulation of bone marrow derived stem cells improves left ventricular function and viability after myocardial infarction.