Aim: The goal of this study was to investigate the value of ST segment resolution after primary percutaneous coronary intervention (PCI) in prediction of infarct size and transmurality measured by cardiac magnetic resonance (CMR) at acute phase. Methods One hundred eight patients who underwent successful primary PCI were evaluated. ST-segment resolution (STR) from all ECG leads related to infarct location was assessed at 90 min and 180 min after primary PCI. CMR was performed at 7.4 짹 5 days after the index procedure. Infarct size and transmurality on delayed-enhancement imaging were measured. Results The STR90min and the STR180min were correlated with infarct size (R=-0.28, p<0.001, and R=-0.43, p<0.001, respectively) and transmurality (R=-0.30, p<0.001, and R=-0.41, p<0.001) of CMR (Figure 1). The receiver-operating characteristic curve identified that the optimal cutoff value of the STR90min to predict large infarct (���25%) was 67% with 75% sensitivity and 60% specificity (AUC=0.69, 95% confidence interval 0.56 to 0.82), and the optimal cutoff value of the STR180min was 71%, with 64% sensitivity and 69% specificity (AUC=0.74, 95% confidence interval 0.61 to 0.87). The STR180min ���70% was associated with non-proximal lesion and non-anterior myocardial infarction, and decreased significantly with a delay of symptom to balloon time and door to balloon time. In a subgroup analysis, the STR180min in patients with non-anterior infarction was not correlated with those CMR parameters, but the STR180min in patients with anterior infarction had correlations with infarct size (R=-0.37, p=0.02) and transmurality (R=-0.41, p=0.02). Conclusion The STR180min ���70%, which is often used as a marker of successful reperfusion after primary PCI for ST-segment elevation myocardial infarction, is good predictor of infarct size and transmurality measured by CMR.