Objective. To investigate whether there is any relation between oxidative stress and the antioxidant system in the development of polycystic ovary syndrome (PCOS) by measuring serum nitric oxide (NO) levels and xanthine oxidase (XO) activity (a generator of reactive oxygen species) and antioxidant status by measuring serum thiol levels and glutathione peroxidase (GSHPx) and paraoxonase 1 (PON1) activities. Design. Prospective casecontrol study. Setting. University hospital in Turkey. Sample. Thirty women with polycystic ovary syndrome and 20 age- and sex-matched healthy control subjects were included. Methods. Serum XO, PON1 and GSHPx activity and NO and thiol levels were determined by spectrophotometric methods. Main outcome measures. Activity of serum XO, PON1 and GSH, as well as NO and thiol levels. Results. Serum XO activities were higher in women with PCOS than in the control women (p<0.001). The PON1 activity was lower in women with PCOS than in the control women (p<0.001). No significant difference was found between NO and thiol levels and GSHPx activities of women with PCOS and the control women (p>0.05). Serum PON1 activities were negatively correlated with serum XO activities and NO levels. Conclusion. Increased oxidant XO activity and decreased lipid antioxidant PON1 activity, along with the observed negative correlation between these parameters, suggests that women with PCOS are under oxidative stress and that there is XO-mediated lipid peroxidation, which may be related to increased atherosclerosis seen in later life in such women.