Background: Distal protection devices are applied during primary PCI to protect distal microvasculature and viable myocardium. It has been shown that delayed hyperenhancement area of MRI correlates well with clinical indices of infarct size. Therefore, we investigated wheather an occlusive balloon type distal protection device(percuSurge GuardWireR; GW) used during primary PCI may reduce infarct size in MRI follow-up. Method: 23 consecutive patients with STEMI (age 55짹11, M:F= 21:2) who underwent PCI were investigated. Patients were classified into three groups: Patients who underwent primary PCI with protection devices within 6 hours from AMI onset (n=5, age 49짹11, group A), those without protection devices within 6 hours from event onset (n=12, age 54짹8, group B) and another group was the patients who underwent elective PCI after the initial 12 hours of symptom onset (n=6, age 63짹13, group C). In all patients, gadolinium contrast enhanced MRI was performed 4-6 days after PCI and 3-4 months after hospital discharge. We measured the mass of infarcted myocardium and left ventricle by tracing of the hyperenhancement area. Absolute values(gram) of mass were calculated using the number of pixels outlined and the slice thickness, assuming a myocardial specific gravity of 1.055g/cm3. We compared the changes of hyperenhancement area between the initial and follow up MRI. Result: There were no statistically significant differences between the groups in initial mass of infarct. The absolute value of infarcted mass and the infarcted mass percent were significantly reduced in follow up MRI (infarcted mass: 31.8짹16.3g->21.6짹9.3g, infarcted mass percent: 24.8짹13.5%->19.8짹10.2%). Absolute reduction of infarcted mass showed tendency to higher in group A (group A: 17.1짹11.8g, group B: 8.6짹4.5g, group C: 7.7짹7.3g, p=0.08), and reduction of infarcted mass percent(infarcted mass/LV mass) were significantly different in those groups(groupA: 29.98짹7.5%, groupB: 18.1짹13.1%, groupC: 4.3짹19.4%, p=0.02) Conclusion: Earlier coronary reperfusion and the use of distal protection device during primary PCI resulted in higher reduction of delayed hyperenhancement infarct size in MRI follow-up.