Primary percutaneous coronary intervention (PCI) is the treatment of choice in patients with ST-elevation myocardial infarction (STEMI) presenting within 12 h of symptom onset. A benefit in the subacute stage is less clear. The aim of the present analysis was to compare myocardial salvage and infarct size between patients with early and late reperfusion after STEMI. We compared cardiac magnetic resonance (CMR) data from a randomized controlled trial (RCT) in STEMI patients presenting within 12 h (n = 695) and a RCT of subacute STEMI patients presenting between 12 and 48 h (n = 93) after symptom onset. CMR imaging was performed 3.9 ± 6.3 days after myocardial infarction. Analyses were performed for an unmatched cohort comprising all patients (n = 788) and a cohort matched for area at risk (n = 186). In the overall cohort, area at risk was similar in both groups [37.1 ± 16.1% of left ventricular mass (%LV) vs. 38.3 ± 16.2%LV; p = 0.50]. Compared to STEMI patients with early reperfusion, patients with late PCI demonstrated larger infarct size (18.0 ± 12.5%LV vs. 28.9 ± 16.9%LV; p < 0.01) and higher extent of microvascular obstruction (1.5 ± 2.9%LV vs. 2.7 ± 4.1%LV; p = 0.01). Myocardial salvage index was significantly smaller in patients with late reperfusion (52.1 ± 25.9 vs. 27.4 ± 26.0; p < 0.01). Analysis of the matched cohorts confirmed the decreased myocardial salvage (p < 0.01) and increased infarct size (p < 0.01) in case of late reperfusion. Compared to patients with timely primary PCI, late reperfusion after STEMI results in decreased myocardial salvage and increased infarct size. However, salvageable myocardium was also found in subacute stages of STEMI.