The purpose of this study was to evaluate the effect of occupational magnetic field exposure on breast cancer in females and to combine residential and occupational magnetic field exposure to reduce misclassification. The study was conducted as a case-control study within a population living within 300 meters of transmission lines in Sweden. We identified cases of breast cancer in females from the national cancer registry, and we selected one matched control per case at random. Residential exposure was estimated through calculations of the magnetic fields generated by power lines. We obtained information about occupation from censuses, and the occupations were linked to a job-exposure matrix that was based on magnetic field measurements. For occupational exposure to magnetic fields over 0.25 microT closest in time before diagnosis, the estimated relative risk was 1.0 [96% confidence interval (CI) = 0.6-1.7]. Women below age 50 years at diagnosis had a relative risk of 1.5 (95% CI = 0.6-3.5). For women below 50 years of age who had estrogen receptor-positive breast cancer, there was a relative risk of 3.2 (95% CI = 0.5-18.9). The results for residential and occupational exposures combined showed similar results.