AbstractObjectiveThe present study was undertaken to compare plasma Se values and glutathione peroxidase (GPX) activity in normal and breast cancer patients.DesignIn a case–control study, forty-five breast cancer patients and the same number of healthy women were randomly selected from their population. Se was measured in plasma by atomic absorption spectrophotometry and GPX activity in erythrocytes was measured using a standard spectrophotometric method.ResultsPlasma Se concentration in healthy women and breast cancer patients was in the normal range, with no statistically significant difference observed between the two groups (138·40 (sd 40·36) μg/l v. 132·15 (sd 35·37) μg/l, respectively). Erythrocyte GPX activity was significantly (P<0·01) higher in breast cancer patients (24·81 (sd 11·66) U/g Hb) compared with healthy women (20·29 (sd 4·24) U/g Hb).ConclusionThe present study indicated that Se deficiency was not a problem in the participants, and sufficient quantity of this element could increase GPX activity to have a protective effect against oxidative damage.