Electric and magnetic fields (EMFs) have been hypothesized to increase the risk of breast cancer, and electric blankets represent an important source of exposure to EMFs. The authors examined the relation between electric blanket use and invasive breast cancer in the Nurses' Health Study. On the biennial questionnaire in 1992, 87,497 women provided information on this exposure during three consecutive time periods. In a prospective analysis with 301,775 person-years of follow-up through 1996 (954 cases), the relative risk for any electric blanket use was not elevated (relative risk (RR) = 1.08, 95% confidence interval (CI): 0.95, 1.24) after controlling for breast cancer risk factors. There was a weak association between breast cancer and electric blanket use at least 16 years before diagnosis and long-term use in age-adjusted analyses but not in multivariate models. In a retrospective analysis of 1,318,683 person-years of follow-up (2,426 cases), the multivariate relative risk associated with use before disease follow-up began was null (RR = 1.05, 95% CI: 0.95, 1.16). Similar results were obtained in analyses stratified by menopause and restricted to estrogen receptor-positive breast cancers. While 95% confidence intervals for these estimates did not exclude small risks, overall, results did not support an association between breast cancer risk and exposure to EMFs from electric blankets.