Background: Randomized trials of distal protection devices did not improve microvascular circulation nor decrease myocardial injury during primary percutaneous intervention (PCI) for ST-elevation myocardial infarction (STEMI). In a prospective randomized trial, we investigated the mechanism of the poor effect of distal protection and thrombus aspiration (DP-TA) in 126 patients with STEMI. Methods: Patients with first-diagnosed STEMI were randomly assigned to DP-TA pretreatment or conventional PCI (c-PCI). End points were infarct size by delayed enhancement (DE), area at risk by T2 high signal (AAR), microvascular occlusion index (MVO) by DE low and T2 iso signal, infarct ratio (infarct size by DE to entire LV size), AAR ratio (AAR to entire LV size), and MVO ratio (MVO to entire LV size, infarct size or AAR) using magnetic resonance imaging (MRI) within 3 days after PCI. Secondary end points were peak CK-MB during hospitalization, ejection fraction (EF) and remodeling index (RI) measured by MRI at post-PCI and 6 months after PCI. Results: Baseline characteristics of the patients including cardiovascular risk factors and lesion characteristics were similar between the two groups. Infarct size, infarct ratio, AAR, MVO and its ratio were not statistically different between DP-TA group and c-PCI group (see table). Mean peak CK-MB, 6-month EF and RI were not different either. Conclusion: DP-TA did not improve any parameter of cardiac MR after primary PCI for STEMI. This study suggests a inherent limitation of DP-TA to remove thromboembolism in STEMI.