To investigate whether hair dye use increases the risk of breast cancer, a case-control study was conducted among patients attending a screening center in New York City. The study group consisted of 398 breast cancer cases identified at the screening center between 1977 and 1981, and 790 randomly selected controls screened during the same period. Subjects were interviewed by telephone to obtain information on known risk factors for breast cancer, along with a complete history of hair dye use detailing type of dye, color, duration, frequency, and temporal periods of use. Most subjects (77%) had used hair dye at least once, 38% of the subjects at least 100 times. However, little increased risk of breast cancer was found among hair dye users. The adjusted odds ratio for ever having used hair dye was 0.8 (95% confidence interval 0.6-1.1), and there was no evidence of a trend in risk with increasing number of hair dye uses. The results were the same whether all past exposures were considered or only exposures more than 10 years before disease. Breast cancer risk did not increase with increasing intensity of exposure, as measured by frequency of use or darkness of color. No effect was seen for different types or colors of dye, or for use during different periods of reproductive life. Although personal hair dye use was unrelated to breast cancer risk, there was an adjusted odds ratio of 3.0 (95% confidence interval 1.1-7.8) for 5 or more years of work as a beautician. Overall, the results of this study, taken in conjunction with the findings of other epidemiologic studies, do not implicate hair dye use as an important cause of human breast cancer.