OBJECTIVES: To assess the oxidative effects of raloxifene use in postmenopausal women by investigating protein carbonyl levels in the plasma. METHODS: Nineteen osteoporotic postmenopausal women treated with raloxifene for 12 months were included in the study. Another seventeen postmenopausal women matched for age and postmenopausal years, without any medication were chosen as a control group. Protein carbonyl levels were determined as oxidative stress markers by the use of Levine's method in the plasma of these women. RESULTS: Serum protein carbonyl levels of postmenopausal women treated with raloxifene (1.27 +/- 0.32 nmol/mg protein) were significantly lower than the control group (2.18 +/- 0.27 nmol/mg protein) (p < 0.05). CONCLUSIONS: Oxidative stress has been found responsible for several diseases including cancer. Protein carbonyl levels, which are the products of protein oxidation, are one of the indicators of oxidative stress. Therefore, the decline in protein carbonyl levels in this study revealed the decreasing oxidative stress. According to our results, it might be interpreted that raloxifene does not cause oxidative stress, and it may even have protective effects in long-term use.