Previous research has demonstrated inconsistent associations between electromagnetic radiation, especially from electric blanket use, and breast cancer. Breast cancer risk according to electric blanket or mattress cover use was examined as part of a multicenter population-based case-control study. Breast cancer patients 50-79 years of age (N = 1949) were identified from statewide tumor registries in Massachusetts, New Hampshire, and Wisconsin from the period June 1994 to July 1995. Women of similar age were randomly selected from population lists as controls. Information regarding electric blanket and mattress cover use and breast cancer risk factors was obtained through telephone interviews. After adjustment for age, body mass index, and other breast cancer risk factors, the risk of breast cancer was similar among ever-users (relative risk = 0.93; 95% confidence interval = 0.82-1.06) and lower among current users than among never-users (relative risk = 0.79; 95% confidence interval = 0.66-0.95). There was no evidence of a dose-response relation with increasing number of months that electric blankets had been used. This study provides evidence against a positive association between electric blanket or mattress cover use and breast cancer.