Background: The presence or absence of ST-segment elevation or depression in the right ventricular (V4R through V6R) and posterior chest leads (V7 through V9) may add diagnostic and prognostic information in the setting of acute coronary syndrome (ACS), predicting high and low rates of in-hospital life-threatening complications, however, the utility, of these additional electrocardiographic (ECG) leads in routine clinical practice has not been fully evaluated.Aim of work: To determine the prevalence of ST segment changes in right precordial and posterior electrocardiographic leads in patients presenting with ACS & having normal or abnormal standard 12-lead ECGs.Methods: We studied 101 patients presenting with chest pain suggestive of acute coronary syndrome. A 12-lead electrocardiogram in addition to 3 right-sided precordial leads (V4R, V5R, V6R) and 3 posterior leads (V7, V8, V9) were recorded on admission. The demographic data of the patients, as well as coronary artery disease risk factors, laboratory and echocardiographic data were collected.Results: One hundred and one patients were enrolled (mean age 56.4±9 years; 77 males and 24 female), 64.4% were smoker and 47.5% were diabetic. The 18 lead ECG showed ST segment changes in 32 (31.7%) patients. The right ventricular leads demonstrated evidence of ST segment elevation in 14 (13.9 %) patients; 7 patients (50%) sustained inferior myocardial infarction (MI) with right ventricular (RV) involvement, and 7 (50%) showed infero-posterior with RV MI, with no isolated ST elevation in right sided ECG, while ST segment depression in the right sided leads was detected in 3 patients (3%). Right coronary artery involvement was more prevalent in this group. The posterior chest leads showed ST segment elevation in 19 patients (18.8%), with two showing isolated posterior ST Elevation. Ten patients presented with posterior and concomitant inferior MI (52.6%) while infero-posterior with RV MI was present in 7 patients (36.8%), with the most diseased artery being the circumflex artery. Patients who had ST segment changes detected by extra chest leads had higher prevalence of segmental wall motion abnormalities (p=0.002), significant mitral regurgitation (p=0.001), and increased right ventricular dimensions (p=0.003). Conclusions: The 18- lead ECG is a reliable and inexpensive tool to identify more patients with posterior and right ventricular myocardial infarction in patients presenting with ACS and signifies a group at a higher risk for developing more significant mitral regurgitation, more segmental wall motion abnormalities and increased right ventricular dimensions.