Background and Methods: Little is known about the role of thrombus aspiration (TA) in NSTEMI with large thrombotic burden. We evaluated the role of TA, its feasibility and outcome for such cases. NSTEMI was defined as clinical myocardial ischemia within 24 hours prior to index visit with elevated serum CK-MB or cardiac troponin-T ���2 upper limit in the absence of persistent ST-segment elevation on 12-lead ECG.Thrombuster II (Kaneka Co., Japan) was used for TA, when TIMI flow grade was 0-1 and/or there were large amount of grossly visible thrombi in infarct related artery. Results: Of 187 NSTEMI patients undergoing PCI in our center during Jan, 2007 to Dec, 2008, TA was required in 37 (20%) retrieving grossly-visible thrombi in 30 (81%). Clinical characteristics of TA group (n=37) were not significantly different from no TA group (n=150), except that men were more prevalent (84% vs. 62%, p=0.01) and peak CK-MB level was higher (137짹142 vs. 60짹77 mg/dL, p=0.003). In TA group, most common culprit was right coronary artery (RCA, 51%), followed by left circumflex or obtuse marginal branch (LCX or OM, 33%) and left anterior descending artery (LAD, 14%), while in no TA group, LAD (36%) followed by RCA (30%) and LCX or OM (22%) (p=0.003). In TA group, pre-PCI TIMI flow was worse (grade 0-1: 73% vs. 31%, p<0.001), and stent was used more frequently (97% vs. 62%, p<0.001). Presumed reasons for being manifested as NSTEMI rather than STEMI despite the presence of thrombotic occlusion in TA group were, TIMI 2-3 antegrade flow (41%), TIMI 0-1 antegrade flow with grade 2-3 collateral flow (35%), and posterior infarction from total occlusion of LCX or RCA (22%). There were no significant differences in final TIMI flow (grade 3: 87% vs. 94%, p=0.16) and MACE (cardiac death, reinfarction or target lesion revascularization) at 12 months (8% vs. 4%, p=0.36) between TA vs. no TA groups. Conclusion: Considerable numbers of thrombotic occlusions usually seen in STEMI are disguised as NSTEMI, presumptively due to good either antegrade or collateral flow, and higher prevalence of posterior infarction not noticed on standard ECG. TA added to PCI for such cases achieves the high rate of thrombus retrieval and may contribute to obtaining comparable final angiographic and clinical outcome.