Chronic exposure to 60-Hz electromagnetic fields has been hypothesized to increase breast cancer risk by suppressing the normal nocturnal rise in pineal melatonin. From 1987 to 1989 in western New York the authors investigated the use of electric blankets as a risk factor for breast cancer in a case-control study of postmenopausal women aged 41-85 years. A study population of 382 cases and 439 randomly selected community controls was queried regarding use in the previous 10 years, the frequency of use by season, and the mode of use. After adjusting for age and education, the odds ratio (OR) for use of an electric blanket in the past 10 years (33% of cases, 35% of controls) was 0.89 (95% confidence interval (95% CI) 0.66-1.19), and the risk did not differ in a dose-response fashion by the number of years used. The risk associated with daily use relative to nonuse was 0.97 (95% CI 0.70-1.35). Use sometimes to warm the bed and sometimes throughout the night was not associated with risk (OR = 0.64, 95% CI 0.40-1.05). The risk of breast cancer among those who used the blanket continuously throughout the night was 1.31 (95% CI 0.88-1.95). Those who reported daily use in season, continuously throughout the night for the past 10 years did not have significantly elevated risk (OR = 1.25, 95% CI 0.73-2.16). Adjustment for the Quetelet index and reproductive risk factors did not alter the results. These findings do not support the hypothesis that electric blanket use is associated with an increased risk for breast cancer. The slightly elevated estimate of risk for the most frequent electric blanket users and the potential public health significance of electromagnetic field exposure suggest that further inquiries be undertaken.