Exposure to 60-Hz magnetic fields may increase breast cancer risk by suppressing the normal nocturnal rise in melatonin. This 1994 Washington State study investigated whether such exposure was associated with lower nocturnal urinary concentration of 6-sulfatoxymelatonin in 203 women aged 20-74 years with no history of breast cancer. Each woman was interviewed and provided data on the following for a 72-hour period at two different seasons of the year: 1) magnetic field and ambient light measured every 30 seconds in her bedroom, 2) personal magnetic field measured at 30-second intervals, and 3) complete nighttime urine samples on three consecutive nights. Lower nocturnal urinary 6-sulfatoxymelatonin level was associated with more hours of daylight, older age, higher body mass index, current alcohol consumption, and current use of medications classified as beta blockers, calcium channel blockers, or psychotropics. After adjustment for these factors, higher bedroom magnetic field level was associated with significantly lower urinary concentration of 6-sulfatoxymelatonin during the same night, primarily in women who used these medications and during times of the year with the fewest hours of darkness. These results suggest that exposure to nighttime residential 60-Hz magnetic fields can depress the normal nocturnal rise in melatonin. It has been suggested that exposure to 60-Hz magnetic fields may increase the risk of breast cancer by suppressing the normal nocturnal rise in melatonin production and release (1), thereby resulting in increased levels of circulating estrogen. Several lines of inquiry have been pursued to investigate a possible link between pineal function, circulating estrogen level, and breast cancer risk (summarized by Stevens and Davis and by Brainard et al. (2, 3)). Other than limited evidence that blood melatonin levels are reduced in human volunteers exposed to magnetic fields (4), there have been few studies of the effect of magnetic field exposure on pineal function in humans. Although some results from experimental and occupational studies suggest that nocturnal melatonin levels can be reduced by exposure to magnetic fields, the evidence thus far is inconsistent and incomplete (5-10). More importantly, it remains unknown whether such exposures can alter the endogenous hormonal environment in women in a manner that might be important in the etiology of breast cancer. Therefore, the present study was undertaken in 1994-1996 to investigate, for the first known time in women, whether exposure to magnetic fields and/or light-at-night is associated with lower nocturnal concentration of the primary metabolite of melatonin found in the urine (6-sulfatoxymelatonin).