The aim of the present study was to assess the levels of oxidative stress markers, catalase (CAT), glutathione peroxidase (GPX) and malondialdehyde (MDA) in severe OSAS and to investigate any correlation between oxidative stress markers and clinical, metabolic and polysomnographic parameters. A total of 30 patients with severe OSAS and 30 healthy controls were included in this cross-sectional, clinical study. Demographic data, polysomnographic, biochemical and clinical indices as well as serum levels of CAT, MDA and GPX were measured and compared in OSAS and control groups. Furthermore, OSAS patients with and without pulmonary hypertension (PHT) were evaluated in terms of levels of CAT, MDA and GPX. Patients with severe OSAS exhibited significantly lower serum levels of CAT (P<0.001) and GPX (P<0.001). Serum MDA levels were remarkably higher in OSAS group (P<0.001). Correlation analysis revealed that levels of CAT and GPX were correlated with apnea-hypopnea index and there was a correlation between serum levels of MDA and CRP. Severe OSAS patients with and without PHT did not reveal any differences for CAT (P=0.789), MDA (P=0.805) and GPX levels (P=0.281). Our results have shown that oxidative stress markers significantly changed in patients with severe OSAS. This information is noteworthy because documentation of the role of oxidative stress in OSAS may have important implications regarding diagnosis, monitoring, treatment and prognosis.