Objective: Elevated serum gamma-glutamyl transferase (GGT) level has been proposed as a risk factor for coronary artery disease and is associated with poor clinical outcome in acute coronary syndrome (ACS). We aimed to evaluate the relationship between GGT level and presence of coronary collateral vessels (CCV) patients with ACS. Methods: We evaluated 178 patients with ACS in this cross-sectional-observational study. Traditional laboratory and clinical parameters and serum GGT levels were measured. All patients underwent coronary angiography on the first day after admission and patients who had >80% stenosis of coronary artery were included in the study. The CCVs graded according to the Rentrop scoring system and Rentrop 0, 1, 2 and 3 were determined in respectively 76 (42.7%), 32 (18.0%), 33 (18.5%), and 37 (20.8%) patients. Rentrop grade 0 was accepted as no CCV development (Group 1), Rentrop grades 1-2-3 were accepted as presence of CCV development (Group 2). Statistical analysis was performed using independent-samples t, Mann-Whitney U and Chi-squared tests, logistic regression and receiver operator curve analyses. Results: Mean age was 62±10 years and 134 (75.3%) of patients were male. Group 1 consisted of 76 (42.7%) patients and Group 2 consisted of 102 (57.3%) patients. The median and minimum-maximum values of serum GGT were 33.5 (8-128) U/L for Group 1 and 23 (2-83) U/L for Group 2. Absence of CCV was significantly associated with high levels of GGT (p<0.001), alanine-aminotransferase (p=0.001), glucose (p=0.011) and low levels of total protein (p=0.020). At multivariate analysis, high levels of GGT were independent predictors of absence of CCV (OR=0.953, 95%CI 0.912-0.996, p=0.031). Conclusion: High levels of GGT on admission were associated with absence of CCV in patients with ACS.