Background. Current evidence clearly indicates that free radicals play a prominent role in the incidence and development of breast cancer. Available literature suggests that tamoxifen is a potent suppressor of lipid peroxide formation in both animal and human systems. The purpose of this study was to understand the rate of lipid peroxidation and the status of antioxidants in tamoxifen‐treated postmenopausal women with breast cancer. Methods. A short term evaluation of 6 months' tamoxifen therapy (10 mg twice a day) in 64 postmeno‐pausal women was conducted. The rate of serum lipid peroxidation and the status of enzymic and nonenzymic antioxidants were evaluated before and after 3 and 6 months' tamoxifen treatment. Results. At 3 and 6 months' evaluation, tamoxifen‐treated patients showed a significantly decreased concentration of malondialdehyde (P < 0.001), an end product of lipid peroxidation, and remarkably increased levels of enzymic and nonenzymic antioxidants. In addition to that, the concentrations of serum selenium and vitamins A, C, and E were increased significantly (P < 0.01 for each) in these patients. Conclusion. The results suggest that tamoxifen therapy exerts significant positive effects on the rate of lipid peroxidation and protective systems in postmenopausal women with breast cancer.