Objective: The prognostic value of the Global Registry of Acute Coronary Events (GRACE) risk score (GRS) and the Thrombolysis In MyocardialInfarction (TIMI) risk index (TRI) has been reported in coronary artery disease (CAD) patients. We aimed to evaluate the relationship betweenthe GRS, TRI, and severity of CAD evaluated by SYNTAX score (SS) in patients with acute coronary syndrome (ACS).Methods: Patients with ACS who were admitted to the coronary care unit of our institution were retrospectively evaluated in this study. A total of 287patients with ACS [154 non-ST elevated ACS (NSTE-ACS), 133 ST elevated myocardial infarction (STEMI)] were included in the study. The GRS andTRI were calculated on admission using specified variables. The severity of CAD was evaluated using the SS. The patients were divided into low(GRS<109)-, intermediate (GRS 109-140)-, and high (GRS>140)-risk groups and group 1 (TRI<17), group 2 (TRI 17-26), and group 3 (TRI>26) accordingto GRS and TRI scores. A Pearson correlation analysis was used for the relation between GRS, TRI, and SS.Results: Patients with a history of coronary artery bypass surgery, those who had missing data for calculating the GRS and TRI, and thosewhose systolic blood pressure (SBP) was more than 180 mm Hg or whose diastolic blood pressure (DBP) was more than 110 mm Hg wereexcluded from the study. Were excluded from the study. There were significant differences in mean age (p<0.001), heart rate (p<0.001), SS(p<0.001), TRI (p<0.001), rate of NSTE-ACS (p<0.001), and STEMI (p<0.001) in all patients between the risk groups. There was a positive significant correlation between the GRS and the SS (r=0.427, p<0.001), but there were no significant correlation between the TRI and SS (r=0.121,p=0.135). The area under the ROC curve value for GRS was 0.65 (95% CI: 0.56-0.74, p=0.001) in the prediction of severity of CAD.Conclusion: The GRS is more associated with SS than TRI in predicting the severity of CAD in patients with ACS.(Anatol J Cardiol 2015; 15: 801-6)