Several previous studies have demonstrated that administration of autologous bone marrow–derived mononuclear cells (BMMNCs) improves cardiac function in patients after acute myocardial infarction (AMI). However, optimum timing of administration has not been investigated in a clinical trial. The Cardiovascular Cell Therapy Research Network was developed and funded by the National Heart, Lung, and Blood Institute to address important questions such as timing of cell delivery and to accelerate research in the use of cell-based therapies. The TIME trial is a randomized, phase II, double-blind, placebo-controlled clinical trial. The 5 member clinical sites of the Cardiovascular Cell Therapy Research Network will enroll 120 eligible patients with moderate-to-large anterior AMIs who have undergone successful percutaneous coronary intervention of the left anterior descending coronary artery and have a left ventricular (LV) ejection fraction ≤45% by echocardiography. Participants will have bone marrow aspirations and intracoronary infusions of 150 × 10⁶ BMMNCs or placebo on day 3 or day 7 post-AMI. Objectives of this study are (1) to evaluate effects of BMMNCs on regional and global LV function compared to placebo therapy in patients with acute AMI as assessed by cardiac magnetic resonance imaging at 6 months and (2) to assess whether effects of BMMNC infusion on global and regional LV function and safety are influenced by the time of administration. This study will provide further insight into the clinical feasibility and appropriate timing of autologous BMMNC therapy in high-risk patients after AMI and percutaneous coronary intervention.