© 2020 Japan Society of Obstetrics and GynecologyAim: We carried out this research to assess thiol disulfide balance in patients with early-stage endometrial cancer. Methods: Fifty-seven endometrial cancer patients and 60 age-matched healthy subjects volunteered for this study. Thiol disulfide parameters and the ratios of these parameters were examined using a colorimetric system. We also evaluated total antioxidant capacity, total oxidant capacity and oxidative stress index. Results: Subjects with endometrial cancer had statistically significantly lower serum native thiol and total thiol levels (224.2 [122.8–267.5] and 270.5 [171.6–323.2], respectively) than healthy subjects (281.35 [213.45–358.9] and 339.55 [274.1–425.95], respectively) (P = 0.001, P < 0.001). Subjects with endometrial cancer had statistically significantly higher serum disulfide/native thiol and disulfide/total thiol ratios (12.22 [8.77–17.61] and 9.82 [7.46–13.02], respectively) than healthy subjects (8.9 [6.79–16.35] and 7.36 [5.9–12.32], respectively) (P = 0.038, P = 0.028). Disulfide/native thiol ratio appeared to be strongly and positively correlated with the stage of endometrial cancer (r = 0.827, P < 0.001). Conclusion: This is an initial report related to thiol disulfide balance in endometrial cancer patients. We believe that oxidative stress contributes both to the evolution and to the progression of the disease. We conclude that deterioration of thiol disulfide balance due to oxidative stress is likely to contribute to the etiopathogenesis of endometrial cancer.