Background: Non ST-segment and ST-segment elevation myocardial infarction (NSTEMI and STEMI, respectively) are on disease spectrum guiding management decision. Some of NSTEMI with large thrombi result in flow limiting coronary lesions similar to those in STEMI. However, little is known about its characteristics and outcome. A comparison is made between flow-limiting NSTEMI versus non-flow limiting NSTEMI and STEMI treated by PCI. Methods: Flow-limiting NSTEMI was defined when the infarct-related artery showed near total or total thrombotic occlusion, or large amount of thrombi requiring aspiration prior to conventional PCI. Outcome was measured by final TIMI flow grade and MACE at 1 year defined as cardiac death, nonfatal infarction, or target lesion revascularization. Results: Of 221 consecutive patients presented to our institution between Dec, 2007 to Jan, 2009 with acute myocardial infarction treated by PCI, NSTEMI was 130 and the prevalence of flow-limiting NSTEMI was 35 (27%) in NSTEMI. Flow-limiting NSTEMI showed similar clinical and angiographic characteristics as STEMI unlike non flow-limiting NSTEMI, in terms of male predominance, less hypertensive, and worse initial TIMI flow and lesion types(Table). Values of peak CKMB and LV ejection fraction of flow-limiting NSTEMI were in between those of STEMI and non flow-limiting NSTEMI, but significantly worse than non flow-limiting NSTEMI. Final TIMI grade and MACE at 12 month were similar between 3 groups. Conclusions: Considerable numbers of NSTEMI present with flow-limiting lesion with large thrombi which behave similarly as STEMI. Early aggressive treatment is necessary to treat this high risk subset of NSTEMI.