 On the basis of experimental studies that bone marrow mononuclear cells BMCs transfer in the injured tissue can promote regional myocardial perfusion and improved cardiac function, several clinical trials have shown that intracoronary bone marrow mononuclear cell BMC transplantation in acute myocardial infarction AMI patients several days after myocardial reperfusion is safe and may enhance the improvement of left ventricular ejection fraction LVEF. The timing of BMC administration, baseline LVEF, dosage of BMC and other factors has been linked to improvement in LVEF after BMC transplantation. In our previous work, we gave BMCs within 24 hours after emergency percutaneous coronary intervention PCI and found that it was safe and effective . In addition, there are another report about longer time from symptom onset to BMC infusion 2-4 weeks, which also appeared effective . The timing of intracoronary stem cell administration may have a critical effect on cell engraftment and may be responsible for the various biological and functional responses to therapy. However, few studies have directly addressed the optimal timing of cell injections. Therefore, in this prospective randomized study, BMCs were given at different times within 24 hours, 3 to 7 days, or 7 to 30 days after reperfusion to investigate whether the timing of therapy affects the therapeutic response of AMI patients. 