Background Because of anatomic location, acute thrombotic occlusion of left circumflex artery often present as non ST-segment elevation on standard electrocardiogram, resulting in deprivation of the opportunity for reperfusion and poor outcomes of the affected patients. Detection of clinical factors associated with thrombotic occlusion of culprit lesion could help identify the subset of those patients who could benefit from prompt reperfusion therapy. Method We studied 276 patients with acute non ST-segment elevation myocardial infarction (NSTEMI) whose culprit lesion was located in the left circumflex artery. Patients with previous history of myocardial infarction were excluded. By reviewing coronary angiogram the patients were classified into 2 groups; those with acute thrombotic occlusion and those without. Clinical, laboratory, electocardiographic, and echocardiographic characteristics were registered for the analyses. Result The acute thrombotic occlusive lesion was found in 94 of 276 (34%) study patients. Demographic characteristics and prevalence of cardiovascular risk factors were not significantly different between two groups. Multivariate analysis by logistic regression revealed accompanying systemic symptoms, non-response to nitroglycerin administration, initial CK-MB ���32 U/mL, ST-segment elevation in lead V6 ��� 0.1mV, and echocardiographic extent of regional wall motion abnormality ��� 2 segments are factors significantly associated with thrombotic occlusion of left circumflex artery. Conclusion In this study, about one third of patients with NSTMI involving left circumflex artery had acute thrombotic occlusive lesion. Further, the present study identified several key factors to be significantly associated with thrombotic occlusion of culprit lesion. Presence of such paramenters suggests the possibility of thrombo-occulsion as underlying pathology, which warrant prompt therapeutic measures of reperfusion in patients with acute NSTEMI.