Abstract Background: Left main coronary artery (LMCA) occlusion is a serious clinical condition. Despite its low incidence, the prognosis is grave. It may present as sudden death, complete heart block, shock and/or acute coronary syndrome (ACS). Aim of Study: The study aimed to investigate the value of ST-segment elevation in lead aVR in the surface ECG as a predictor of LMCA lesions in patients suffering from non-ST-segment elevation myocardial infarction (NSTEMI) and its relation to the severity of LMCA. Material and Methods: It is a retrospective study including patients admitted to CCU in Banha University Hospital, Shebin El-Kom Teaching Hospital and National Heart Institute. A total number of 100 patients with the diagnosis of NSTEMI ACS who had significant LMCAD on coronary angiography were included in this study; the patients were classified according to the presence or absence of ST-segment elevation of >!0.05mV in lead aVR into two groups: - Group 1: Patients with isoelectric ST-segment in lead aVR or with ST-segment elevation of less than 0.05mV. - Group 2: Patients with ST-segment elevation of 0.05mV or more in aVR. Results: Patients with elevated ST segment more than 1 is significantly higher among patients who died than those who are still alive (p<0.001). Twenty eight patients (37.8%) had Left Main coronary disease in group 2, less than 80% stenosis compared to 18 (75%) patients in group 1. While 46 patients (62,2%) had Left Main stenosis more than 80% In group 2, compared to 6 patients (25%) in group 1, the differences between subgroups were statistically signif-icant difference (p-value <0.001). Conclusion: ST-segment elevation in lead aVR is a sen-sitive prognostic indicator in detecting left main coronary artery lesions of patients suffering from non-ST-segment elevation myocardial infarction (NSTEMI).