Objective: To determine the serum thiol/disulphide homeostasis in preeclampsia and to investigate the association with clinical parameters. Methods: Forty-three pregnant women with preeclampsia and 43 healthy, uncomplicated pregnancies were included in the study. Native thiol, disulphide, and total thiol concentrations were measured by a novel automated method. Results: The serum native and total thiol levels were significantly lower in preeclampsia when compared with the healthy group (p<0.001). There were significant correlations among the homeostatic parameters and preeclampsia. Conclusion: Serum thiol/disulphide homeostasis may have a role in the pathogenesis and can be evaluated with the clinical and laboratory findings of preeclampsia.