Breast cancer is the leading cause of cancer death in women in the industrialized world, and the rates of breast cancer incidence are rising. Although risk is high in industrialized societies, it is low in nonindustrialized areas. The search for the causes of breast cancer has not yet yielded a convincing explanation for the geographic and temporal patterns in the occurrence of breast cancer. Generation of electric power is a hallmark of industrialization, and two products of electric power, light-at-night (LAN) and electromagnetic fields (EMF), may affect breast cancer risk. Exposure to either LAN or EMF can decrease production of melatonin by the pineal gland. Melatonin, in turn, has been shown to suppress mammary tumorigenesis in experimental animals. Moreover, recent epidemiological findings indicate an increased risk of breast cancer in workers occupationally exposed to EMF. On the basis of these considerations, it is proposed that the use of electrical power accounts, in part, for the higher risks of breast cancer in industrialized societies.