Background and Purpose: A recent study showed the distal protection device improved neither ventricular function nor infarct size after primary angioplasty in acute myocardial infarction (AMI). Contrast-enhanced magnetic resonance (CE-MR) imaging may be sensitive enough to demonstrate the benefit of the distal protection device. The purpose of this study was to evaluate the efficacy of distal protection device using CE-MR during primary angioplasty. Methods: From September 2003 to May 2004, 22 patients who underwent primary angioplasty due to ST segment-elevation AMI within 12 h after symptom onset were randomized to either PercuSurge GuardWire system (PS group) or no protection (control group). All patients underwent CE-MR before discharge (6짹6 days after procedure). Steady-state and 5-minute delayed CE-MR images were acquired to determined left ventricular ejection fraction (LVEF), regional wall thickening, regional wall motion and the volume of delayed hyperenhancement. Results: There were no significant differences in the baseline clinical and angiographic characteristics between 2 groups. The procedures were successful without complications in all patients. The PS group was associated with a significantly higher incidence of myocardial blush score 3 compared to control group (PS 73%, control 36%, p = 0.005). There was no difference in the LVEF, regional wall motion and systolic thickening. The volume of delayed contrast hyperenhancement (DCH), however, was significantly smaller in the PS group. The incidence of clinical event was similar between 2 groups during the follow-up period.Conclusions: Distal protection device may be useful to reduce the infarct size after primary angioplasty in the AMI. CE-MR may be useful tool to demonstrate the infarct size reduction in this setting.