To define causes and pathological mechanisms underlying differences in clinical outcomes, we compared the findings of contrast-enhanced magnetic resonance imaging (CE-MRI) between ST-segment elevation myocardial infarction (STEMI) and non-ST-segment elevation myocardial infarction (NSTEMI). In 168 patients undergoing early invasive intervention for STEMI (n=113) and NSTEMI (n=55), CE-MRI was performed a median of 6 days after the index event. Infarct size was measured on delayed-enhancement imaging, and area at risk (AAR) was quantified on T2-weighted images. The median infarct size was significantly smaller in the NSTEMI group than in the STEMI group (10.7% [5.6-18.1] versus 19.2% [10.3-30.7], P<0.001). Although there was a trend toward a greater myocardial salvage index ([AAR - infarct size] �� 100/ AAR) in the NSTEMI group compared to the STEMI group (48.2 [30.4-66.8] versus 40.5 [24.8-53.5], P=0.056), myocardial salvage index was similar between the groups in patients with anterior infarction (39.6 [20.0-54.9] versus 35.5 [23.2-53.4], P=0.96). The NSTEMI group also had a significantly lower extent of microvascular obstruction and a smaller number of segments with >75% of infarct transmurality relative to the STEMI group (0% [0-0.6] versus 0.9% [0-2.3], p<0.001 and 3.0짹2.3 versus 4.6짹2.9, P=0.001, respectively). Myocardial hemorrhage was detected less frequently in the NSTEMI group than the STEMI group (22.6% versus 43.8%, P=0.029). In the multivariate analysis, baseline Thrombolysis In Myocardial Infarction flow grade 3 and hemorrhagic infarction were closely associated with ST-segment elevation (OR 0.32, 95% CI 0.13-0.81, P=0.017; OR 5.66, 95% CI 1.77-18.12, P=0.003, respectively). In conclusion, in vivo pathophysiological differences revealed by CE-MRI assessment include more favorable infarct size, AAR, myocardial salvage and reperfusion injury in patients with NSTEMI compared to those with STEMI undergoing early invasive intervention.