Background : The state of coronary microcirculation is an important determinant of myocardial viability and clinical outcomes in acute myocardial infarction (AMI). However, there has been lacking in comparative studies on the most reliable invasive assessing the microvascular integrity and the effect of thrombus aspiration in AMI. This study is to investigate whether the manual thrombus aspiration improves the index of microcirculatory resistance (IMR) and clinical outcomes during primary percutaneous coronary intervention (PCI) for ST-segment elevation myocardial infarction (STEMI). Methods : We collected clinical, biochemical and angiographic information in 45 consecutive patients (41 men and 4 women; age 55 짹 10 years) who underwent primary PCI for STEMI from May, 2009. Before PCI, STEMI patients were randomly assigned to standard PCI (n = 21) or thrombus aspiration PCI (n =24). After PCI, We assessed IMR, coronary flow reserve (CFR) by intracoronary Pressure wire. All patients underwent echocardiography before discharge after primary PCI. Results : There were no significant differences in reperfusion time (onset to balloon time) and CFR (1.83 짹 0.7 vs 2.30 짹 1.6, p=0.335), ejection fraction (47.1 짹 8.1% vs 44.7 짹 8.2%, p = 0.435) between two groups. There was significant difference in IMR between two groups (36.0 짹 15.0 vs 25.5 짹 8.4, p=0.027). Conclusions : It is well known that the IMR appears to be a good predictor of microvascular damage after STEMI. The effect of thrombus aspiration might improve microvascular integrity in patients with STEMI.