Obstructive sleep apnea (OSA) patients present increased cardiovascular morbidity and mortality. Oxidative stress is involved in OSA and cardiovascular pathology. The aim of the study was to assess oxidative stress markers in the blood of OSA males during oral glucose tolerance test (OGTT). The study involved OSA-suspected obese males (BMI ≥ 25 kg/m(2)) aged 35-64, with no acute or chronic disorders, appointed for polysomnography to diagnose OSA (AHI ≥ 5). The results of OGTT allowed to select prediabetic (Pre) subjects and normal glucose tolerance (N), excluding newly diagnosed diabetes. Blood was collected at 0 min (fasting) and 120 min of the test. Plasma glucose, total antioxidant status (TAS), thiobarbituric acid-reacting substances (TBARS), and activity of superoxide dismutase-1 (SOD) in erythrocytes, were determined at the two time points and the difference (D) between the 120 and 0 min time points was calculated for either oxidative stress variable (D-TAS, D-SOD and D-TBARS). Fasting serum insulin and lipids also were measured fasting. There were four groups of subjects, each consisting of 22 individuals N-OSA-neg(ative), N-OSA-pos(itive), Pre-OSA-neg and Pre-OSA-pos. The N-OSA-pos and Pre-OSA-pos subjects demonstrated decreased SOD-0 compared with OSA-negative groups. In N-OSA-neg and N-OSA-pos groups, the positive differences D-SOD and D-TAS were observed, while Pre-OSA subjects presented negative differences. In conclusion, prediabetic OSA patients may consume blood antioxidant factors to counter the effects of oxidative stress, more than individuals with normal glucose tolerance.