Objective: The gamma-glutamyl cycle catalyzed by gamma-glutamyl transferase (GGT) plays an important role in glutathione (GSH) homeostasis inthe cell. In cells continuously exposed to the drug, the main phase of the enzymatic detoxification is the conjugation of the drug with GSH catalyzed byglutathione-S-transferase (GST). Conjugation of drugs with GSH is the first step in the development of chemotherapeutic drug resistance. In this study,we aimed to investigate the relationship between GGT and GSH in molecular subgroups of breast cancer patients.Materials and Methods: Serum GGT activity and GSH levels for patients diagnosed with breast cancer (n=58) and healthy controls (n=8) weremeasured by a spectrophotometric method and a colorimetric kit, respectively.Results: GGT activity was significantly higher in the total patient group and in the molecular subgroups than those in the control groups (p<0.05).Serum GSH levels were higher in the patient groups compared to controls without reaching statistical significance (p>0.05). GGT activity was positivelycorrelated with GSH levels in the total patients and healthy controls (p<0.001 and p<0.05, respectively). There was also a positive correlation betweenGGT activity and GSH levels in Luminal A, HER2-positive (Human epidermal growth factor receptor 2), and Triple-negative groups (p<0.05).Conclusion: This is the first study showing the relationship between GGT and GSH in molecular subgroups of breast cancer. An increase in GGTactivity may affect intracellular GSH synthesis. Therefore, having a correlation between GGT and GSH in some molecular subgroups may affect thecourse of treatment in these patients.