The lifetime risk of breast cancer is similar among Black and white women in the U.S., but Black women are disproportionately affected by aggressive breast cancer subtypes such as estrogen receptor (ER) negative tumors, which are diagnosed at a younger age and have a higher mortality rate. While certain hair care products, including relaxers (straighteners) and leave-in conditioners, used more commonly by Black than white women may contain compounds with estrogens or endocrine-disrupting chemicals, few epidemiologic studies have assessed the relationship of hair relaxer use to breast cancer risk.