Oxidative stress has been suggested to play a role in some physiological conditions and in many disease processes, including carcinogenesis. The aim of the study was to investigate the association between the oxidative stress and selenium levels in the erythrocytes of breast cancer patients at different clinical stages. The extent of lipid peroxidation was assessed by measuring thiobarbituric acid reactive substances (TBARS), called malondialdehyde (MDA), in erythrocytes. Glutathione peroxidase activity (GPx) in erythrocyte and serum selenium levels were determined in patients and controls. Thirty-three female breast cancer patients at different clinical stages were divided into four groups. Ten control subjects were also included in this study. Atomic absorption, spectrophotometry, and colorimetric methods were used to obtain serum selenium and erythrocyte MDA levels, respectively. GPx activities were assessed with kinetic method described by Paglia and Valentine (J Lab Clin Med 1967; 70(1):158-169). All cancer patients (groups A, B, and C) showed significant increases in erythrocyte GPx activities compared with those in control subjects (P < 0.05). No significant differences were found among the cancer patients at different clinical stages (P > 0.05). Although plasma selenium levels did not change among the groups A, B, and C, it was demonstrated that breast cancer patients have lower serum selenium levels than healthy controls (P < 0.05). Significantly increased lipid peroxidation, measured as erythrocyte MDA, was demonstrated in patients with breast cancer compared to the control group (P < 0.05). There were no differences between group A and group B in terms of MDA levels (P > 0.05). Erythrocyte MDA levels were significantly lower (P < 0.05) in breast cancer patients who had had surgery within 1 month (group A) when compared with those levels in breast cancer patients who had just been diagnosed and had not had any surgery yet (group C). Although clinical stage in breast cancer does not have any effect on serum selenium levels or erythrocyte GPx activity, erythrocyte MDA levels changed among the patients at different clinical stages. Increased MDA levels in untreated patients show us increased oxidative stress. If we removed malignant breast tissue by surgery and gave adjuvant therapy, lipid peroxidation product MDA levels in erythrocyte would decrease. Increased GPx enzyme activity in erythrocytes may be a result of a protective mechanism that develops in breast cancer patients against free radical damage. Although an inverse correlation between the serum selenium levels and erythrocyte GPx activity in patients with breast cancer is suggested, there is no correlation at different clinical stages of patients.