The aim of the study was to determine some biochemical factors reflecting oxidant-antioxidant balance in the blood of patients being in different stages of the obstructive sleep apnea syndrome (OSAS). The patients, suspected of OSAS, were submitted to biochemical and somnographic examinations. APolyMesam device was used to establish the severity of apnea episodes. The control group was composed of subjects without OSAS. The following parameters were determined: concentration of plasma total antioxidant status, TAS; activity of erythrocyte Cu, Zn-superoxide dysmutase, SOD; plasma lipid peroxidation products, TBARS. The OSAS patients had lower levels of SOD and TAS, and increased level of TBARS. We conclude that oxidative stress markers increase in the blood of OSAS patients to a variable degree, depending on the severity of the syndrome and, therefore, may be suitable for a clinical follow-up of such patients.