BACKGROUND: Primary percutaneous coronary intervention(PCI) is currently considered the gold standard treatment in patients with ST-segment elevation myocardial infarction(STEMI). However, distal embolizations of atheromathrombotic material often have occurred during PCI in STEMI, compromising optimal myocardial reperfusion. Several studies demonstrated that thrombus aspiration in patients with STEMI results in better reperfusion and clinical outcomes than conventional PCI. The purpose of this study is to investigate whether mechanical thrombus aspiration may be always necessary in primary PCI. METHODS: This study was a single-center, non-randomized, retrospective trial. Between September 1, 2006 to July 31, 2008, 154 consecutive patients with STEMI who underwent a primary PCI were enrolled in this study. The control group consisted of 70 treated patients who underwent conventional PCI without thrombus aspiration. The thrombus aspiration group consisted of 84 treated patients who underwent PCI with a stent accompanied by thrombus aspiration. RESULTS: The coronary flow disturbance was much more in the control group(16.2%, 12/74) than in the thrombus aspiration group(3.8%, 3/80)(p=0.013). If the pain to balloon time was more than 12 hr, the rates of flow disturbance was significantly higher in the control group than the thrombus aspiration group(50% vs 0%, p=0.007). However, there was no significant differences between the control group(11.6%) and the thrombus aspiration group(7.3%) in case of pain to balloon time less than 12 hr. CONCLUSION: Thrombus aspiration in early reperfusion therapy might be unnecessary. However, the mechanical thrombus aspiration before stenting in case of doing delayed reperfusion therapy should be considered.