Aim: To investigate the role of dynamic thiol-disulfide homeostasis in preeclamptic and idiopathic fetal growth restricted (FGR) pregnancies. Material and method: In this prospective case-control study, a total of 110 singleton pregnancies with FGR (study group) (51 preeclamptic and 59 idiopathic FGR's cases) were compared with 68 healthy pregnant controls at the same gestational weeks (control group). For serum disulfide-thiol homeostasis, a newly used method described by Erel and Neselioglu was used. Results: Serum native thiol and total thiol levels were lower in FGR pregnancies (285.63 +/- 55.92 mu mol/L, 324.41 +/- 44.18 mu mol/L, respectively) than control group (324.41 +/- 44.18 mu mol/L, 362.98 +/- 51.43 mu mol/L, p < .001, p = .004, respectively). In subgroup analysis, only preeclamptic FGR's have lower native and total thiol levels (254.41 +/- 59.55, 324.41 +/- 44.18 mu mol/L, respectively) compare to both idiopathic FGR's and control's. There was no difference in native and total thiol levels with idiopathic FGR's with controls. Idiopathic FGR's have higher levels of disulfide than preeclamptic FGR's (21.72 +/- 17.72 versus 16.80 +/- 11.20 mu mol/L). The serum albumin and total protein levels were positively and spot urine protein/creatinine ratio, 24-h urine protein levels were negatively correlated with native thiol and total thiol levels. Conclusion: The balance of thiol-disulfide homeostasis was shifted and native and total thiol levels were decreased only in preeclamptic FGR pregnancies. The serum disulfide level was increased in idiopathic FGR pregnancies compare to preeclamptic FGR pregnancies which may be a sign of oxidative stress in idiopathic FGR pregnancies with normal thiol pool.