in recent years. Thiol and disulphide play an important role in cell signal mechanisms, antioxidant protection, and detoxification. In this study, we aimed to evaluate the role of Thiol-Disulphide homeostasis (TDH) in colorectal cancer (CRC) by using a new method. Material and Method: The patients (pts) who diagnosed with CRC and healthy control subjects were included to study. Serum samples for the thiol-disulphide test were obtained at the time of diagnosis. TDH tests were measured by the automated spectrophotometric method by describing Erel and Neselio glu. Thiol-disulphide homeostasis was also evaluated according to tumor stage and localization. Results: Eighty-eight pts with CRC and 110 control were enrolled. Native thiol (NT), disulphide and total thiol (TT) levels were significantly lower in patients compared with the control arm (Median NT: 402 e424, p ¼ 0.003; median Disulphide 18.7e21, p ¼ 0.011; median TT: 437e467, p ¼ 0.001). Thiol/disulphide balance was also maintained (p ¼ 0.149). TT and NT levels were not differed according to tumor localization whereas disulphide level was significantly higher in left-sided tumors than right-sided (19.9 e13.07 respectively, p ¼ 0.007). In addition, disulphide/NT ratio was also significantly higher in left-sided than right-sided (0.1e0.12 respectively, p ¼ 0.015). Thus, the balance of dynamic TDH is disrupted in favor of disulphide between left-sided and right-sided tumor. There was also no significant difference between thiol-disulphide levels and tumor stage whereas thiol level tends to lower in stage 4 disease (p ¼ 0.7). Conclusion: This is the first trial that evaluates the relationship with dynamic TDH and CRC according to tumor stage and localization. Thiol and disulphide may play an important role in the pathogenesis of CRC.