Background— Intracoronary administration of autologous bone marrow–derived mononuclear cells (BM-MNC) may improve remodeling of the left ventricle (LV) after acute myocardial infarction. The optimal time point of administration of BM-MNC is still uncertain and has rarely been addressed prospectively in randomized clinical trials. Methods and Results— In a multicenter study, we randomized 200 patients with large, successfully reperfused ST-segment elevation myocardial infarction in a 1:1:1 pattern into an open-labeled control and 2 BM-MNC treatment groups. In the BM-MNC groups, cells were administered either early (ie, 5 to 7 days) or late (ie, 3 to 4 weeks) after acute myocardial infarction. Cardiac magnetic resonance imaging was performed at baseline and after 4 months. The primary end point was the change from baseline to 4 months in global LV ejection fraction between the 2 treatment groups and the control group. The absolute change in LV ejection fraction from baseline to 4 months was −0.4±8.8% (mean±SD; P =0.74 versus baseline) in the control group, 1.8±8.4% ( P =0.12 versus baseline) in the early group, and 0.8±7.6% ( P =0.45 versus baseline) in the late group. The treatment effect of BM-MNC as estimated by ANCOVA was 1.25 (95% confidence interval, −1.83 to 4.32; P =0.42) for the early therapy group and 0.55 (95% confidence interval, −2.61 to 3.71; P =0.73) for the late therapy group. Conclusions— Among patients with ST-segment elevation myocardial infarction and LV dysfunction after successful reperfusion, intracoronary infusion of BM-MNC at either 5 to 7 days or 3 to 4 weeks after acute myocardial infarction did not improve LV function at 4-month follow-up. Clinical Trial Registration— URL: http://www.clinicaltrials.gov . Unique identifier: NCT00355186.