Aim: The aim of our study is to compare serum thiol/disulphide homeostasis of women diagnosed with abortus imminens (AI) and healthy pregnant women, and to determine whether it has a role in the pathogenesis of the disease or not.Materials and methods: A total of 100 pregnant women were included in the study. The study group consisted of 50 patients with AI whereas 50 healthy pregnant women were chosen as the control group. All of the patients in the two groups were matched for age, gestational age, and body mass index. Thiol/disulphide levels were analyzed with a newly developed automated spectrophotometric method.Results: We found significantly reduced levels of native thiol (SH) (370.235.2mol/l versus 397.6 +/- 29.3mol/l) and total thiol (406 +/- 37.1mol/l versus 434.5 +/- 29.9mol/l), in the sera of the study group compared to the control group (p<.001). But serum levels of disulphide (SS) were statistically insignificant between the two groups (17.9 +/- 5.9mol/l versus 18.4 +/- 4.9mol/l, p: .612). Similarly, no significant differences were observed between the two groups in terms of SS/SH, SS/total thiol and SH/total thiol ratios (all p>.05).Conclusions: We found a decrease in thiol levels which have antioxidant properties in patients with AI. Decreased antioxidant level in the body is thought to play a role in the etiology of AI. In this regard, further studies are needed to elucidate the potential role of dynamic thiol/disulphide homeostasis in the pathogenesis of AI.