In their report, Loomis et al. described an excess of female breast cancer among certain "electrical" occupations that they postulate was due to extremely low frequency electromagnetic field exposure. The greatest excess risk was found in telephone installers, repairers, and line workers who, as a group, had an adjusted odds ratio of 2.17 for mortality for breast cancer relative to other employed women. We suggest problems exist in studies that do not investigate the presence of confounding exposures and that use job titles as surrogates for worker exposures. Loomis et al. discount the existence of ionizing radiation exposure in these groups of workers. We have recently completed a study of a telephone central office facility where workers install and maintain lines and switches for the telephone company. We found that the cross-bar switching machinery, historically used in the central office facility, contained vacuum tubes having at least 1 microCi of radium bromide and were located in racks holding 60 tubes per rack. While these cross-bar switches have been replaced by more modern equipment that do not utilize radium bromide tubes, they were still in use in at least one central office facility as late as 1992. Of importance to the issue of breast cancer was the finding that central office facility workers, who may be included in the category "installers, repairers, and line workers," may have carried these tubes in their shirt pockets. The telephone company estimated the dose rate from these tubes to be about 4 mR/h at the point of bodily contact (3). The role of this potential exposure to a well-established carcinogenic agent in the development of male and female breast cancer among central office facility workers has not yet been evaluated; however, it is not unrealistic to assume that it may be far greater than whatever cancer-inducing effect is postulated by the inhibition of melatonin from exposure to electromagnetic fields. If the category of telephone installers, repairers, and line personnel (as used by Loomis et al.) includes central office facility workers, then our findings suggest the presence of a major confounder - ionizing radiation. For telephone pole workers, exposures to extremely low frequency electromagnetic fields may not be different from those in other occupations. Means of limited exposure levels to extremely low frequency electromagnetic fields among telephone installers, repairers, and line (pole) personnel measured by the National Institute for Occupational Safety and Health (NIOSH) investigators ranged from 1.3 to 14.8 mG. The highest value was obtained from a worker who was using a gasoline-powered drill for 2-minute intervals, with levels of exposure up to 718 mG that could not be attributed to exposure to either telephone wires (carrying 48-V DC current) or overhead 60-Hz power lines. Because the workers in prior years used hand drills, the use of the gas drill is relatively new. With the exception of the use of the drill, no mean measurement of worker exposure exceeded 4.5 mG; these values are supported by a study conducted by telecommunications industry research (4). In this study, the extremely low frequency electromagnetic field exposure levels were measured in the same group of workers cited by Loomis et al. These values approximate those that NIOSH investigators have found in office settings, where sporadic exposure to electrical devices such as pencil sharpeners, computers, fans, and other equipment similarly skews the mean values upward (5,6). The overall exposure to extremely low frequency electromagnetic fields of telephone installers, repairers, and line workers may not be any higher than that of many other occupational groups of workers.