Hair relaxers (straighteners) have been used by millions of African American women, often for many years. Relaxer ingredients can enter the body through scalp lesions and burns. Because manufacturers are not required to list all ingredients, these products may contain unknown harmful substances. We assessed hair relaxer use in relation to breast cancer incidence in the Black Women's Health Study, a nationwide follow-up study of African American women. Information on hair relaxer use was collected in 1997; 48,167 women were followed subsequently through 2003 for incident breast cancer. Incidence rate ratios of breast cancer were estimated with Cox regression. During 266,298 person-years of follow-up, 574 incident cases of breast cancer were ascertained. There were no increases in breast cancer risk associated with any categories of duration of hair relaxer use, frequency of use, age at first use, number of burns experienced during use, or type of relaxer used. The incidence rate ratio for use at least seven times a year for 20 or more years relative to use for less than a year was 0.98 (95% confidence interval, 0.78-1.39). These results suggest that hair relaxer use does not increase the incidence of breast cancer in African American women.