An epidemiologic case-control study of 1617 patients with a primary breast cancer and 1617 control subjects was conducted to test the hypothesis that use of hair dyes is related to breast cancer. No overall association was observed between breast cancer risk and "ever use" of hair dyes (odds ratio, 1.04; 95% confidence interval, 0.90 to 1.21), age when hair dye use started and age when it stopped, duration of hair dye use (years), types of hair dyes used, and estimated lifetime number of individual applications. The current data set also failed to show an increased risk for breast cancer in women who had been diagnosed with benign breast disease and were exposed to hair dyes.