Background Prompt reperfusion in patients with ST-segment elevation myocardial infarction (STEMI) reduces infarct size and improves survival. However, the intuitive link between infarct size and prognosis has not been convincingly demonstrated in the contemporary era. Objectives This study sought to determine the strength of the relationship between infarct size assessed early after primary percutaneous coronary intervention (PCI) in STEMI and subsequent all-cause mortality, reinfarction, and hospitalization for heart failure. Methods We performed a pooled patient-level analysis from 10 randomized primary PCI trials (total 2,632 patients) in which infarct size was assessed within 1 month after randomization by either cardiac magnetic resonance (CMR) imaging or technetium-99m sestamibi single-photon emission computed tomography (SPECT), with clinical follow-up for ≥6 months. Results Infarct size was assessed by CMR in 1,889 patients (71.8%) and by SPECT in 743 patients (28.2%). Median (25th, 75th percentile) time to infarct size measurement was 4 days (3, 10 days) after STEMI. Median infarct size (% left ventricular myocardial mass) was 17.9% (8.0%, 29.8%), and median duration of clinical follow-up was 352 days (185, 371 days). The Kaplan-Meier estimated 1-year rates of all-cause mortality, reinfarction, and HF hospitalization were 2.2%, 2.5%, and 2.6%, respectively. A strong graded response was present between infarct size (per 5% increase) and subsequent mortality (Cox-adjusted hazard ratio: 1.19 [95% confidence interval: 1.18 to 1.20]; p < 0.0001) and hospitalization for heart failure (adjusted hazard ratio: 1.20 [95% confidence interval: 1.19 to 1.21]; p < 0.0001), independent of age, sex, diabetes, hypertension, hyperlipidemia, current smoking, left anterior descending versus non-left anterior descending infarct vessel, symptom-to-first device time, and baseline TIMI (Thrombolysis In Myocardial Infarction) flow 0/1 versus 2/3. Infarct size was not significantly related to subsequent reinfarction. Conclusions Infarct size, measured by CMR or technetium-99m sestamibi SPECT within 1 month after primary PCI, is strongly associated with all-cause mortality and hospitalization for HF within 1 year. Infarct size may, therefore, be useful as an endpoint in clinical trials and as an important prognostic measure when caring for patients with STEMI.