Germline mutations of breast cancer-associated gene 1 (BRCA1) predispose women to breast and ovarian cancer. It was recently shown that bilateral oophorectomy decreases breast cancer incidence in BRCA1 mutation carriers. To model human BRCA1 carriers, our laboratory has previously created mice with a conditional knockout of the full-length BRCA1 gene in the mammary epithelium combined with a heterozygous knockout of the p53 tumor suppressor gene. These mice developed ER-negative mammary tumors and were employed to determine the effects of oophorectomy on tumor formation. Individual knockout mice (BRCA1(Co/Co) MMTV-Crep53+/-), following two complete pregnancies, were either oophorectomized or sham treated. Mice were subsequently examined for the development of palpable mammary tumors until they were 12 months of age. Until 135 days post-oophorectomy (255 days of age), the tumor incidence was similar in both oophorectomized and intact mice, approximately 30%. After this time, the increase in tumor incidence was much lower in the oophorectomized mice, while tumor incidence increased in non-oophorectomized mice. The effects of oophorectomy on mammary development in both control and knockout mice were also examined. Oophorectomized mice with a conditional knockout of full-length BRCA1 in conjunction with a loss of one p53 allele exhibited glandular regression with a reduction in the number of mammary epithelial cells following oophorectomy. This study employed a model that may be relevant for testing agents useful against breast cancer in BRCA1 carriers and a subset of sporadic cancers. The data also show that oophorectomy, if performed significantly prior to the time that tumors arise, appears to be quite effective.