Aim: Obstructive sleep apnea syndrome (OSAS) is a chronic and incapacitating disease that often requires lifelong care. This study aimed to evaluate the thiol/disulfide homeostasis in patients with OSAS, to compare the thiol/disulfide levels with the control group and to investigate their relationship with the severity of the disease. Material and methods: Patients who were admitted to the department of chest diseases, and diagnosed with OSAS using polysomnographic analysis (n = 186) and 144 patients who underwent polysomnography due to some reasons but ruled out of having OSAS were included in the study. Serum total thiol (TT), native thiol (SH), and disulfide thiol (SS) levels were measured from the participants; SS/SH, SS/TT, and SH/TT percent ratios were calculated and compared between the patient and control groups. Results: The mean (+/- SD) age of the patients and control participants was 52.0 +/- 11.5 years and 44.9 +/- 13.2 years, respectively. Compared to the control group, patients with OSAS had significantly lower SH (239.3 +/- 56.3 mu mol/L vs. 258.6 +/- 65.3 mu mol/L, t = 2.70, p =.007) and TT levels (273.2 +/- 60.1 mu mol/L vs. 292.9 +/- 67.5 mu mol/L, t = 2.64, p=.010). Age (OR = 1.04), serum albumin (OR = 12.67), ischemia-modified albumin (IMA) (OR = 0.12), SH (OR = 0.81), and TT (OR = 1.17) were independent predictors of OSAS. Conclusions: These results support the idea that decreased ST and TT levels are related to increased oxidative stress. On the other hand, impaired thiol balance may play a significant role in the pathogenesis of OSAS.