OBJECTIVES This study was performed to evaluate the impact of time to reperfusion on infarct size and transmural extent after primary percutaneous coronary intervention (PCI) for ST-segment elevation myocardial infarction (STEMI). BACKGROUND The relationship between duration of ischemia and extent of myocardial injury is not well known. METHODS In 60 patients undergoing primary PCI for STEMI, contrast-enhanced magnetic resonance imaging was performed. Infarct size and transmural extent on delayed-enhancement imaging were measured. RESULTS Infarct size was not associated with increase in symptom onset-to-balloon time (29.0짹10.6% for <180 min, 26.7짹10.8% for 180 to 360 min, and 29.9짹13.1% for >360 min, p=0.65) and door-to-balloon time (26.3짹8.5% for <90 min, 30.6짹11.1% for 90 to 120 min, and 27.9짹14.6% for >120 min, p=0.46). Transmural extent of infarct increased significantly with increasing delay in symptom onset-to-balloon time (73.5짹22.2% for <180 min, 79.0짹13.3% for 180 to 360 min, and 90.4짹8.2% for >360 min, p=0.01), but not with door-to-balloon time (80.0짹14.8% for <90 min, 76.5짹20.1% for 90 to 120 min, and 85.1짹14.2% for >120 min, p=0.30). In multivariate analysis, anterior infarction independently predicted large infarct (infarct size ���35%) [odds ratio 11.9, 95% confidence interval (CI) 1.44–98.70, p=0.02]. The independent predictors of high transmurality (average transmural extent ���75%) were symptom onset-to-balloon time (odds ratio per 30 min 1.34, 95% CI 1.06–1.69, p=0.01) and the use of glycoprotein IIb/IIIa inhibitors (odds ratio 0.05, 95% CI 0.01–0.35, p=0.003). CONCLUSIONS In patients undergoing primary PCI for STEMI, symptom onset-to-balloon time affected transmural extent of infarct rather than infarct size.