Background: The measurement of left ventricular (LV) ejection fraction (LVEF) is a strong predictor of cardiovascular adverse events and mortality in patients with LV dysfunction and has become the most common primary end point in cardiovascular cell therapy trials after ST-segment elevation myocardial infarction (STEMI). Multiple small trials have been performed using bone marrow mononuclear stem cells (BMCs) in this setting with several meta-analyses demonstrating that BMC administration results in a small improvement in LVEF and may attenuate adverse LV remodeling. However, individual trial results have not been uniform, and the measurement of LVEF in these trials has relied on a variety of imaging techniques including LV angiograpnhy, single-photon emission computed tomography, echocardiography, or cardiac magnetic resonance imaging (cMRI). Methods: Because cMRI provides the most accurate measurement of LVEF, LV volumes, and infarct size in patients after STEMI, we reviewed all randomized cardiovascular stem cell trials (N = 10) that administered intracoronary BMCs versus placebo/control to 686 patients after primary percutaneous coronary intervention treatment of STEMI that used cMRI as their principal imaging measurement of LVEF at baseline and 3 to 6 months later. Results: Administration of BMCs was associated with a nonsignificant 0.9% ± 0.8% absolute increase in LVEF compared with placebo or control (95% CI -0.7 to 2.4) with a small but nonsignificant decrease LV end-diastolic and LV end-systolic volumes (LV end-diastolic volume -1.1 ± 1.5 mL/m 2, LV end-systolic volume -1.6 ± 1.4 mL/m 2). Although infarct size uniformly decreased over time, the reduction was not improved by BMC administration (-0.3 ± 1.7 g). Conclusions: The benefit of BMC administration after STEMI on LVEF, LV volumes, and infarct size is small when assessed by cMRI.