Aim: To investigate the effects of severity of preeclampsia on thiol-disulfide homeostasis (TDH).Material and methods: A total of 108 participants were divided into three groups: Group 1 was composed of pregnant women with no obstetric complications, Group 2 included pregnant women with mild preeclampsia, and Group 3 consisted of pregnant women with severe preeclampsia. TDH parameters were determined, and comparisons of clinical and routine laboratory test findings were made in all groups.Results: The serum native thiol level was 347.927.4 in the control group, 237.2 +/- 44.2 in the mild preeclampsia group, and 227.9 +/- 53.1 in the severe preeclampsia group (p<0.001). The serum total thiol level was 376.1 +/- 31.9 in the control group, 261.8 +/- 49.4 in the mild preeclampsia group, and 248.3 +/- 57.4 in the severe preeclampsia group (p<0.001). The disulfide level was 14.1 +/- 5.6 in the control group, 12.3 +/- 5.1 in the mild preeclampsia group, and 10.2 +/- 4.8 in the severe preeclampsia group (p=0.001). A significant correlation between impairment in degree of TDH and severity of preeclampsia was observed.Conclusion: TDH was impaired in women with preeclampsia, and this impairment increased with disease severity. Therefore, impaired TDH may have a role in the etiopathogenesis of the disease.