Background: Primary percutaneous coronary intervention (PCI) is effective in opening the infarct-related artery in patients with myocardial infarction with ST-segment elevation (STEMI). And thrombus aspiration (TA) before angioplasty, guided by angiographic selection criteria, is better effective in reperfusion and clinical outcomes than conventional PCI in patients with STEMI. However, it has been unknown whether TA itself or direct stenting after TA results in improved myocardial reperfusion. Methods: All consecutive patients presenting with acute myocardial infarction with ST-segment elevation were considered for participation in this study. We performed a randomized trial assessing whether manual aspiration without predilation is superior to conventional aspiration with use of predilation in patients with TIMI 2 or 3 flow achieved via TA during primary PCI. Epicardial and myocardial patency were assessed according to the TIMI (thrombolysis in myocardial infarction) flow scale and TIMI myocardial perfusion (TMP) grade angiographically. Clinical outcome was assessed using MACE (cardiac death, MI, target lesion revascularization). The index of microcirculatory resistance (IMR) was used for assessing the status of the microcirculation independently of the epicardial area. An intracoronary pressure/temperature sensor-tipped guide was introduced after PCI and thermodilution curves was obtained at baseline and during maximal hyperremia achieved by infusion of intravenous adenosine. Results: A total of 31 patients have been randomly assigned to the direct stenting (DS group) or stenting after balloon predilation (PB group) during 6 months. There were no significant differences in baseline characteristics, including age (59.1짹9.5 vs 58.4짹12.3), sex, traditional risk factors, pain-to-door and door-to-balloon time, use of Glycoprotein IIb/IIIa Inhibitor, and target coronary artery between DS (n=15) vs. PB (n=16) groups. Although final TIMI flow (grade 3: 93% vs. 82%, p=0.14), TMP (grade 3: 76% vs. 64%, p=0.19), and peak CK-MB level (178짹90 vs. 154짹104 mg/dL, p=0.25) were comparable between DS and PB group, post-procedural IMR was significantly lower (28.43짹12.83 vs. 38.63짹29.45 mmHg∙sec, p=0.04) in DS group than PB group. Conclusion: The DS without predilation after aspiration thrombectomy during primary PCI is safe and more effective strategy to improve myocardial perfusion than conventional aspiration thrombectomy. This preliminary report lends the support to the use of this treatment as a standard approach during the TA of acute myocardial infarct favored by the low cost.