The purpose of this study was to use cardiac magnetic resonance (CMR) imaging to define the morphology of the reversibly-injured peri-infarct border zone in patients treated with primary percutaneous coronary intervention (PPCI) for acute ST-elevation myocardial infarction (STEMI). In fifteen patients T2-weighted myocardial edema imaging was used to identify the ischemic bed or Area-at-Risk (AAR), and late gadolinium enhancement imaging was used to measure infarct size. The images were co-registered and the boundary of edema and necrosis were defined using an edge-detection methodology. We observed that infarction always involved the subendocardium but showed variable transmural extension within the AAR. The mean infarct size was 22 ± 19% (range 8 - 48%) and the mean AAR was 34 ± 12% (range 20 - 57%). The infarcted myocardium was always smaller than the ischemic AAR and involved between 34 - 99% (mean 72 ± 21%) of the ischemic bed primarily due to variation in transmural infarct extension. Although a lateral border zone of potentially viable myocardium was often present its extent was limited (range 0 - 11mm; mean 5 ± 4mm). As a result of this infarcts occupied the majority (range 70 - 100%; mean 82 ± 13%) of the width of the AAR. The mean fractional wall thickening in the infarcted, peri-infarct and remote myocardium was 3.6 ± 16.0%, 40.5 ± 26.4% and 88.2 ± 39.3% respectively. These findings demonstrate that myocardial salvage is largely determined by epicardial limitation of the infarct within the ischemic AAR following PPCI. The lateral boundaries of necrosis approximate to the lateral extent of the ischemic bed and systolic wall motion abnormalities extend well beyond the infarct border zone.