The relationship between occupational exposure to 60 hertz magnetic fields and the risk of breast cancer in women was examined. Case information covering the years 1988 through 1991 was obtained from the state cancer registries of Maine, Wisconsin, Massachusetts, and New Hampshire. Controls were randomly selected from the population. Altogether, 6,888 cases and 9,529 controls were interviewed. Exposure to magnetic fields was categorized as background, low, medium, or high according to occupation. Equal percentages of cases and controls worked outside the home. Compared to the background exposure group, the breast cancer odds ratios (ORs) adjusted for age, state, and breast cancer risk factors were 1.43 for the high exposure group, 1.09 for the medium exposure group, and 1.02 for the low exposure group. The OR for premenopausal women in the high exposure group was 1.98, relative to the premenopausal women of the background exposure group. For the premenopausal women of the medium and low exposure groups, the ORs were 0.82 and 0.91, respectively. The OR for postmenopausal women in the high exposure group was 1.33, relative to the postmenopausal women of the background exposure group. Among the postmenopausal women of the medium and low exposure groups, the ORs were 1.10 and 1.01, respectively. In the high exposure group, an OR of 1.79 was calculated for computer equipment operators, whereas an OR of 1.20 was calculated for electric/electronic equipment assemblers. In the medium exposure group, an OR of 7.99 was determined for precision inspectors, testers, and related workers. However, this value was based on only nine cases and two controls. The authors conclude that a modest relationship may exist between occupational exposure to 60 hertz magnetic fields and breast cancer risk among women.