The aim of the study was to assess the markers of oxidant-antioxidant status in excess body mass index (BMI) persons with and without the obstructive sleep apnea syndrome (OSAS). In overweight (BMI-1: 25.0-29.9 kg/m(2)) control and OSAS and obese (BMI-2: 30.0-34.9 kg/m(2)) control and OSAS subjects with no acute or chronic disorder the following markers were determined: concentration of plasma total antioxidant status, TAS, (Randox); activity of erythrocyte Cu, Zn-superoxide dysmutase, SOD, (Randox); plasma concentration of thiobarbituric acid reacting substances, TBARS, (Yagi method). BMI-1-OSA presented decreased SOD, compared with the BMI-1-C group (P=0.006). BMI-2-OSA showed both SOD (P=0.002) and TAS (P=0.047) decreased, and elevated TBARS (P=0.03) compared with the BMI-2-C group. There was a positive correlation between TAS & SOD and a negative one between TAS & TBARS in the BMI-1-C group. In BMI-2-C, a negative correlation between TAS & TBARS was observed. We conclude that OSAS decreases the blood antioxidant status in high BMI persons and may change the relationship between oxidative stress markers.