Recent experimental and clinical trials give rise to the hope that progenitor cells could replace scar tissue after myocardial infarction with healthy functional myocardium. However, while a significant increase in left ventricular ejection fraction has been described after progenitor cell transplantation in several clinical trials, long-term results are lacking, and the mechanisms underlying the improvement of ejection fraction are unclear. Therefore, the efficacy of progenitor cell transplantation after myocardial infarction has not been established, and potential problems may have been underestimated. In-depth laboratory and animal studies are needed to determine the best cell type, optimal amount of cells, and time point for transplantation. Treatment of patients with progenitor cells outside well controlled prospective trials should be avoided.