BACKGROUND: Serial cardiac magnetic resonance imaging (CMR) is the reference standard for evaluating left ventricular function, wall motion, and infarct size in patients with acute myocardial infarction, as well as remodeling during follow-up. The cardiac CMR substudy of the randomized multicenter REPAIR-AMI trial (Reinfusion of Enriched Progenitor cells And Infarct Remodeling in Acute Myocardial Infarction study) aimed at gaining insight into postinfarction left ventricular remodeling processes. METHODS: Consecutive patients with ST-segment elevation myocardial infarction and primary percutaneous coronary intervention were enrolled (n = 204) and randomly assigned to either stem cell therapy (bone marrow-derived progenitor cells [BMC]) or placebo after bone marrow aspiration. In the magnetic resonance imaging substudy, 54 patients completed serial CMR (baseline, 4 and 12 months, respectively) after enrollment (27 BMC, 27 placebo). Image analysis was performed at a central core laboratory. RESULTS: There were no significant differences between the 2 groups with respect to global ejection fraction (EF), end-diastolic volume (EDV), and end-systolic volume (ESV) at baseline. At 12 months, the treatment effect of BMC infusion on EF amounted to 2.8 absolute percentage points (P = .26), the progression of EDV at 12 months was less in the BMC group (treatment effect 14 mL, P = .12), and unlike placebo, ESV did not increase (absolute treatment effect 13 mL, P = .08), respectively. In patients with a baseline EF < median (EF < or = 48.9%), BMC administration was associated with a significantly improved EF (+6.6%, P = .01), reduced EDV increase (treatment effect 29.1 mL, P = .02), and abrogation of ESV increase (treatment effect 29.4 mL, P = .01) after 12 months, respectively. CONCLUSION: Intracoronary administration of BMC additionally improved left ventricular function in patients with impaired left ventricular function after ST-segment elevation myocardial infarction despite optimal "state-of-the-art" reperfusion and pharmacologic treatment on 1-year follow-up and beneficially interfered with adverse postinfarction left ventricular remodeling.