AIMS: We sought to determine whether repeat administration of bone marrow mononuclear cells (BMC) can improve left ventricular function compared with a single infusion in patients with large acute myocardial infarction (AMI). METHODS AND RESULTS: Thirty-nine patients with a ST-elevation AMI of the anterior wall and a significantly decreased left ventricular ejection fraction (LVEF 20–39%) were randomly assigned to three groups following primary percutaneous coronary intervention: Group A (n=12) received a single intracoronary infusion of BMC (1.9 ± 1.2 × 10) at 3–7 days after AMI; Group B (n=15) received BMC administration both at 3–7 days (2.0 ± 1.4 × 10) and at 3 months (2.1 ± 1.7 × 10); and the control group (CON, n=12) received one placebo injection at 3–7 days. We noted no severe complications associated with the BMC transfer. The increase in LVEF evaluated by magnetic resonance imaging (MRI) after 12 months in Group B (11.7 ± 2.6%) was significantly greater than that in Group A (7.2 ± 1.6%, P < 0.001) or in CON (2.9 ± 2.0%, P < 0.001). Magnetic resonance imaging-derived myocardial infarct size decreased significantly in Group B compared with Group A (11.3 ± 2.7% vs. 6.3 ± 1.6%, P < 0.001). CONCLUSION: Data from this preliminary study suggest that repeated BMC administration might be a safe and feasible therapeutic strategy for patients with large AMI.