A few studies have noted moderate elevation in the risk of breast cancer among women with residential exposure to electromagnetic fields, among women without a history of much strenuous physical activity, and among women with in utero exposures that may indicate high levels of maternal estrogen. The relative risk for each of these associations has generally been less than 2, with little adjustment for possible confounding factors. Also, several studies have not been able to confirm these relations. Currently there is scant or no evidence that silicone breast implants or psychological factors increase the risk of breast cancer. Despite the limited number of studies, the inconsistent results, and the difficulty of measuring several of these exposures, the plausible biologic mechanisms for each indicate that more research on these hypotheses is warranted.