Background: Repetitive episodes of hypoxia and reoxygenation during sleep in patients with obstructive sleep apnea syndrome (OSAS) resemble an ischemia-reperfusion injury. We aimed to test the hypothesis that oxidative stress occurs in pregnant women with OSAS. We also aimed to compare thiol/disulfide homeostasis with ischemia-modified albumin (IMA) and total antioxidant capacity (TAC) as markers of ischemia-reperfusion injury in pregnant women with and without OSAS and healthy control. Methods: This study included 29 pregnant women with OSAS, 30 women without OSAS in the third trimester applying for periodic examinations, and 30 healthy women. Serum IMA and TAC (using the ferric reducing power of plasma method) were measured. Serum thiol/disulfide homeostasis was determined by a novel automated method. Results: The mean age of the pregnant women with OSAS was 31.0 +/- 4.7 years with a mean gestational age of 36.5 +/- 3.0 weeks. The mean age of pregnant women without OSAS was 29.8 +/- 4.9 years with a mean gestational age of 36.9 +/- 2.7 weeks. The mean age of the nonpregnant control group was 29.7 +/- 6.4 years. Both native thiol (291 +/- 29 mu mol/L versus 314 +/- 30 mu mol/L; p = .018) and total thiol (325 +/- 32 versus 350 +/- 32, p = .025) levels were lower in pregnant women with OSAS compared to pregnant women without OSAS, respectively (p < .01). Conclusions: This is the first study demonstrating the thiol/disulfide homeostasis in pregnant women with OSAS. Native thiol and total thiol levels were lower in pregnant women with OSAS compared to those without OSAS. However, dynamic thiol/disulfide homeostasis parameters cannot provide valuable information to discriminate OSAS in pregnant women.