The objective of this study was to determine thiol/disulphide homeostasis (TDH) in infertile cows with subclinical endometritis (SCE). Endometrial cytological samples were collected using a cytobrush to diagnose SCE in 36 infertile cows. According to the results of the cytology examination, those with acute endometritis were classified as Group I (n = 20) and those with chronic endometritis were classified as Group II (n = 16). A control group was formed of heifers as Group III (n = 20). Blood samples were taken from each group on the day of diagnosis (day 0) to analyse TDH. In the cytology examination, both the Giemsa method and immunocytochemical staining were applied to determine chronic inflammation and activity status. In 55.55% (20/36) of the infertile cows with cytological endometritis, the inflammation was determined to be active, and in 44.44% (16/36) it had become chronic. The native thiol and total thiol levels were found to be statistically significantly lower in the acute (206.54 +/- 8.30 mu mol/L; 227.11 +/- 9.30 mu mol/L) and chronic SCE cases (225.15 +/- 11.89 mu mol/L; 247.96 +/- 10.80 mu mol/L) compared to the heathy control group (308.47 +/- 13.59 mu mol/L; 336.83 +/- 15.5 mu mol/L respectively) (P<0.001). Disulphide levels, disulphide/total thiol, native thiol/total thiol and disulphide/native thiol ratios were similar in all the groups (P>0.05). The diagnostic accuracy of native thiol, which can be used in the diagnosis of SCE, was 92.8%, that of total thiol was 89.3% and that of disulphide was 64.3% according to the ROC curve analysis. These results demonstrate that TDH is a reliable and sensitive indicator of oxidative stress in cow SCE, and that abnormal TDH might play a role in SCE pathogenic mechanisms. This is the first study to evaluate thiol/disulphide homeostasis in dairy cows with SCE as a new indicator of oxidative stress.