Non-ST elevation myocardial infarction (NSTEMI) severity is difficult only with electrocardiogram (ECG). In most cases, NSTEMI patients are followed with cardiac enzymes without early invasive intervention if no severe chest pain exists in the emergency department (ED) or coronary intensive care unit (CICU).Our aim was to evaluate association between CAD severity and ST segment change in lead aVR (STaVR)/T wave amplitude in lead aVR (TAaVR) ratio in patients with NSTEMI.We included 306 patients with NSTEMI in the ED between 2015 and 2016. STaVR and TAaVR values were measured from 12-derivation ECG underwent on admission to the ED. The absolute values of STaVR and TAaVR were calculated and the following were obtained; ratio 1:|STaVR|/|TAaVR|, ratio 2:|TAaVR|/|STaVR|, and ratio 3: by dividing the variable with larger absolute value to other variable with smaller absolute value (|larger value|/|smaller value|). The SYNTAX score (SS) was calculated from coronary angiography in all patients.In analysis of the bivariate correlation between SS and ratios; significantly positive, strongly with ratio 3 (r = 0.692, P < .001), and only ratio 3 was determined to be an independent predictor for SS in linear regression analysis (OR: 0.642, 95% CI: 0.432-0.853, P = .001).Severity of CAD may be estimated by evaluating STaVR and TAaVR ratio in patients with NSTEMI.