Recently, the influence of free radicals and lipid peroxides on many diseases, the effect of sex hormones on lipid peroxidation and antioxidant effects of estrogens have received considerable interest. In the present study we aimed to investigate the relationship between sex hormones and both lipid peroxidation and glutathione content in women with polycystic ovary syndrome (POS), in healthy women and in healthy men. We measured levels of lipid peroxides and sex hormones in plasma and levels of glutathione in erythrocytes of all cases. We evaluated the level of thiobarbituric acid reactive substances (TBARS) as an index of lipid peroxides and erythrocyte glutathione level as an index of antioxidant. We found that plasma levels of free testosterone, dehydroepian-drosterone sulfate (DHEAS) and estradiol significantly higher in the women with POS group than in the healthy women group. There was no significant difference in the levels of both plasma TBARS and erythrocyte glutathione, between women with POS group and healthy women group. Plasma DHEAS levels of healthy men and women with POS were similar. Plasma TBARS level was higher and erythrocyte glutathione level was lower in the healthy men group than in both the healthy women group and in the women with POS group. These data imply that testosterone has an oxidant effect. DHEAS which is an antioxidant, has a protective role in females with POS. Estrogens have an antioxidant effect but this action changes according to its dominant degradation pathway.