Aim: Controversy surrounds the hypothetical relationship between low serum levels of selenium and reduced activity of selenium-dependent enzymes, such as glutathione peroxidase, and an increased risk of cancer in humans. This study investigated serum concentrations of selenium in women with and without breast cancer. Methods: In this case-control study, we compared serum concentrations of selenium in women with breast cancer (n = 200), healthy women (n = 100), and women with chronic diseases (n = 100). Patients with breast cancer were divided into premenopausal (n = 99) and postmenopausal subjects (n = 101). Results: Mean serum concentrations of selenium were 81.1 µg/l in women with breast cancer and 98.5 µg/l in women with non-tumoral disease (p < 0.001). Conclusion: Alterations in serum concentrations of selenium in women with breast cancer appear to be a consequence, rather than a cause of cancer. In accordance with the hypothesis, the findings suggest that very low selenium status could be due to the nature of cancer.