BACKGROUND Early ST segment elevation resolution after successful primary PCI in myocardial infarction (MI) is related with lesser myocardial microvascular damage after angiographically successful restoration of coronary flow. But pathologic basis of early ST segment restoration after primary PCI in is not clear. We tried to identify the pathologic change of early ST resolution after PCI with late gadolinium enhancement cardiac magnetic resonance (CMR) infarction. METHODS fifty eight patients (49 male, mean age: 58 years) who underwent angiographically successful [Thrombolysis in Myocardial Infarction (TIMI III)] primary PCI and taken CMR within 7 days was enrolled. Forty-four patients of them were taken follow-up CMR after 3 mon. The patients were classified based on reduction in ST segment elevation in an ECG in 90 min after the PCI compared to immediate before intervention (group 1: ��� 50%, group 2: < 50 %). Left ventricular function, dimension, infarct mass and volume were quantified in three arterial territories. RESULTS ST segment elevation resolution (Group 1) showed 28 patients (63.6 %). Both group had similar angiographic results, initial ejection fraction (EF), initial infarct mass and volume. Infracted thickness during CMR follow-up was significantly smaller in group 1 (p = 0.017). Also group 1 had a significant reduction of infracted thickness and volume during CMR follow-up (p < 0.05). CONCLUSIONS Early ST segment elevation resolution after successful primary PCI demonstrated significant infracted volume and thickness reduction in CMR. This finding proved early ST segment resolution is related to the reduction of infarct territory with CMR.