Background: Primary percutaneous coronary intervention(PCI) for ST-segment elevation acute myocardial infarction(STEMI) is effective in opening the infarct-related artery, restoring epicardial flow and improving clinical outcomes. However, atherothrombotic embolization during PCI induces microvascular obstruction and reduces myocardial tissue reperfusion. Methods: We analyzed 118 consecutive patients who underwent primary PCI for STEMI at Ulsan University Hospital from January 2004 to June 2008. Among these patients, manual thrombus aspiration was routinely performed during primary PCI in 52 patients(44%) since July 2006. The primary endpoint was 70% ST-segment elevation resolution (STR) measured 90 min after PCI by electrocardiography. Secondary end points included corrected TIMI frame count(CTFC). Result: Baseline clinical and angiographic characteristics were similar in the 2 groups.(mean age: 57.3짹12.5, male: 77%). Complete STR occurred in 54% of the patients in the thrombus aspiration group and in 30% of those in the standard PCI grou(P=0.01). Corrected TIMI frame count significantly improved in patients with thrombus removal in comparison with standard PCI: 20.2짹9.9 frames vs 29.7짹18.8 frames(p <0.001). Conclusion: Manual thrombus aspiration in unselected patients with STEMI undergoing primary PCI is clinically feasible and results in better angiographic and myocardial reperfusion than standard PCI.