Prophylactic bilateral salpingo-oophorectomy is recommended for BRCA1 mutation carriers to prevent ovarian cancer. Whether or not hormone replacement therapy (HRT) initiated after oophorectomy is associated with an increased risk of breast cancer has not been evaluated in a prospective study. To determine the association between HRT use and BRCA1-associated breast cancer. A prospective, longitudinal cohort study of BRCA1 and BRCA2 mutation carriers