Objective: We aimed to investigate the oxidative stress status in a population of women with untreated major depressive disorder. Methods: Fifty-four female patients with untreated major depressive disorder and 68 female healthy controls were included in the study. A Sociodemographic Form, the Hamilton Anxiety Rating Scale, and the Hamilton Depression Rating Scale (HAM-D) were applied to all the participants. Fasting blood samples were collected from all participants to assess serum thiol/disulphide levels and their pairwise ratios. Results: Native thiol levels were significantly higher and disulphide levels were lower in patients as compared to controls, while total thiol levels were not significantly different between the groups. Disulphide/native thiol and disulphide/total thiol ratios were significantly lower, while the native thiol/total thiol ratio was significantly higher, in the patient group than the control group. There was a negative correlation between HAM-D score and disulphide level, disulphide/native thiol ratio, and disulphide/total ratio, while there was a positive correlation between HAM-D score and native/total thiol ratio, in the patient group. Discussion: This is the first study to investigate dynamic thiol/disulphide homeostasis in women with untreated major depressive disorder. Our results showed dynamic thiol/disulphide homeostasis shifts towards thiol formation which implies an antioxidant reaction in women with untreated major depressive disorder. [Anadolu Psikiyatri Derg 2018; 19(2.000): 135-142]