Thyroid cancer disproportionally affects more women than men. The aim of this study was to assess whether exposure to extremely low frequency electric magnetic fields from electric blankets (EBs) was associated with the development of thyroid cancer. Data were analyzed from 89,527 women who participated in the Women's Health Initiative Observational Study and who responded to questions concerning prior use of EBs. During a mean follow-up of 12.2 years, 190 incident cases of thyroid cancer were identified. We estimated the hazard ratio (HR) and 95 percent confidence interval (CI) of incident thyroid cancer associated with EB use by Cox's proportional hazard model, adjusted for selected covariates. A majority, 57 percent, of the women in the cohort reported the use of EBs while sleeping and/or for warming the bed before sleep. No association was found between use of EBs and subsequent risk of thyroid cancer (HR = 0.98, 95 percent CI 0.72-1.32). Duration of EB use measured in years, months, or hours had no effect on risk. These results did not change when the cases were limited to papillary thyroid cancer, the most frequently occurring histologic type. The results of this study do not support possible health hazards of EBs in regards to thyroid cancer risk.