Background. Gamma glutamyl transferase (GGT) is involved in the pathophysiologic process of coronary atherosclerosis. GGT activity plays a role in the catabolism of glutathione which is known as one of the major antioxidants. However, there is a lack of research on direct examination of relevance between serum GGT activity with systemic oxidative stress. Objectives. We aimed to investigate the relationship between GGT activity with systemic oxidative stress markers and the extent and complexity of coronary artery disease (CAD) assessed with SYNTAX score in stable CAD. Methods. Measurements were obtained from 359 patients with stable CAD (Mean age = 57.7 +/- 10.1 years). The patients were divided into two groups according to the median GGT level (GGT < median group < 22 and GGT > median group > 22). Angiography was performed and SYNTAX score was calculated in all patients. Oxidative stress markers (total oxidant status [TOS], total antioxidant capacity [TAC] and oxidative stress index [OSI]) were measured in all patients. Results. While SYNTAX score and oxidative stress markers such as TOS and OSI have been increased, TAC was decreased in GGT > median group compared with GGT < median group (p < 0.05, for all). GGT activity was independently associated with diabetes (beta = 0.106, p = 0.015) and OSI (beta = 0.556, p < 0.001) in multiple linear regression analysis. However, the independent association between GGT activity and SYNTAX score was not found in present study (beta = 0.063, p = 0.238). Conclusion. In stable CAD, increased GGT activity within the normal range is associated with increased oxidative stress rather than increased extent and complexity of CAD.