Background: Symptom-to-balloon time (SBT) and door-to-balloon time (DBT) are both considered important metrics in patients undergoing primary percutaneous coronary intervention (pPCI) for ST-segment–elevation myocardial infarction (STEMI). We sought to assess the relationship of SBT and DBT with infarct size and microvascular obstruction (MVO) after pPCI. Methods: Individual patient data for 3115 ST-segment–elevation myocardial infarction patients undergoing pPCI in 10 randomized trials were pooled. Infarct size (% left ventricular mass) was assessed within 1 month after randomization by technetium-99 m sestamibi single-photon emission computerized tomography (3 studies) or cardiac magnetic resonance imaging (7 studies). MVO was assessed by cardiac magnetic resonance. Patients were stratified by short (≤2 hours), intermediate (2–4 hours), or long (>4 hours) SBTs, and by short (≤45 minutes), intermediate (45–90 minutes), or long (>90 minutes) DBTs. Results: Median [interquartile range] SBT and DBT were 185 [130–269] and 46 [28–83] minutes, respectively. Median [interquartile range] time to infarct size assessment after pPCI was 5 [3–12] days. There was a stepwise increase in infarct size according to SBT category (adjusted difference, 2.0% [95% CI, 0.4–3.5] for intermediate versus short SBT and 4.4% [95% CI, 2.7–6.1] for long versus short SBT) but not according to DBT category (adjusted difference, 0.4% [95% CI, −1.2 to 1.9] for intermediate versus short DBT and −0.1% [95% CI, −1.0 to 3.0] for long versus short SBT). MVO was greater in patients with long versus short SBT (adjusted difference, 0.9% [95% CI, 0.3–1.4]) but was not different between patients with intermediate versus short SBT (adjusted difference, 0.1 [95% CI, −0.4 to 0.6]). There was no difference in MVO according to DBT. Results were similar in multivariable analysis with SBT and DBT included as continuous variables. Conclusions: Among 3115 patients with ST-segment–elevation myocardial infarction undergoing infarct size assessment after pPCI, SBT was more strongly correlated with infarct size and MVO than DBT.