BACKGROUND: Although some studies found improved coronary flow and myocardial salvage when stent implantation was deferred, the DANAMI-3-DEFER (Third DANish Study of Optimal Acute Treatment of Patients With ST-elevation Myocardial Infarction) did not show any improvement in clinical outcome in patients with ST-segment elevation myocardial infarction (STEMI) treated with primary percutaneous coronary intervention (PCI) and deferred stenting. OBJECTIVES: This study sought to evaluate the effect of deferred stent implantation on infarct size, myocardial salvage, and microvascular obstruction (MVO) in patients with STEMI. METHODS: In the present DANAMI-3 substudy, a total of 510 patients with STEMI were randomized to PCI with deferred versus immediate stent implantation. The patients underwent a cardiac magnetic resonance examination before discharge after the index procedure and again 3 months later. The primary endpoint was final infarct size. RESULTS: Deferred stenting did not reduce final infarct size (9% left ventricle [LV]; interquartile range [IQR]: 3% to 18% vs. 10% LV; IQR: 3% to 18%; p = 0.67). Similarly, deferred stenting was not associated with myocardial salvage index (66%; IQR: 50% to 89% vs. 67%; IQR: 49% to 88%; p = 0.80) or presence of MVO (43% vs. 42%; p = 0.78). In a post hoc analysis, stent length was the only subgroup of 7 that had an effect on outcome. In patients with a stent length ≥24 mm, deferred stenting reduced the final infarct size (6% LV; IQR: 2% to 18% vs. 13% LV; IQR: 7% to 23%; p = 0.006; and p for interaction = 0.005). CONCLUSIONS: In the DANAMI-3-DEFER cardiac magnetic resonance substudy, routine deferred stenting did not reduce infarct size or MVO and did not increase myocardial salvage. These results do not support the use of routine deferred stenting in STEMI patients treated with primary PCI. (DANish Study of Optimal Acute Treatment of Patients With ST-elevation Myocardial Infarction [DANAMI-3]; NCT01435408).