In this first detailed, prospective analysis of HRT use by formulation type after oophorectomy and breast cancer risk in BRCA1 mutation carriers, we reported cumulative incidence of breast cancer of 22% among women who used estrogen plus progesterone HRT compared to 12% among women who used estrogen-alone HRT (P-log rank=0.04). Here, BRCA1 is linked to breast carcinoma.