Alterations in the expression of the breast and ovarian cancer susceptibility gene BRCA1 may contribute to the development of mammary and ovarian neoplasia. The sex-steroid estrogen modulates cell proliferation of normal and neoplastic breast and ovarian epithelial cells, but the role of estrogen regulation on the expression of BRCA1 remains to be defined. In this study, estrogen-regulated BRCA1 expression was examined in breast and ovarian cancer cells. Estrogen stimulated the proliferation of estrogen receptor (ER)-positive breast MCF-7, C7-MCF-7, and ovarian BG-1 cells as well as the expression of the estrogen-inducible pS2 gene. This was concomitant with upregulation of BRCA1 mRNA (2.5- to 5.0-fold) and a 3- to 10-fold induction of BRCA1 protein (230 kDa). Cell fractionation studies localized the BRCA1 protein to the nucleus in both unstimulated and estrogen-stimulated cells. The antiestrogen ICI-182780 inhibited estrogen-induced cell proliferation, BRCA1 mRNA induction, and BRCA1 protein expression in ER-positive cells. Conversely, estrogen did not influence expression of BRCA1 in HBL-100 cells that lacked the estrogen receptor, although the constitutive levels of BRCA1 mRNA (but not protein) in these cells were 5- to 30-fold higher than in other breast and ovarian cancer cells. Secretion of the BRCA1 protein into the cell medium did not account for the discrepancy between the mRNA and protein levels in HBL-100 cells. Proliferation of HBL-100 cells was not affected by either estrogen or ICI-182780. Taken together, these data support a role for the steroid estrogen and the involvement of the estrogen receptor pathway in the modulation of expression of BRCA1. We therefore propose that stimulation of cell proliferation may be a prerequisite for upregulation of BRCA1 in breast and ovarian cancer cells.