Breast cancer is a hormone-based disease with numerous factors contributing to the lifetime risk of developing the disease. While breast cancer risk is reduced by nearly 50% after one full term pregnancy, women over the age of 25 have a significantly greater risk of developing breast cancer immediately following parturition compared to their nulliparous counterparts. It is widely presumed that the increased risk of developing breast cancer following pregnancy is due to the ability of pregnancy-associated hormones to promote the further proliferation of an initiated target cell population. It is surprising however, that the majority of breast cancers that develop following pregnancy lack appreciable expression of either the estrogen or progesterone receptors. This important observation suggests that if hormones play a part in promoting breast cancer following pregnancy, they may not be doing so through direct binding to hormone receptor molecules expressed by breast cancer cells. To reconcile this conceptual conflict we investigated the hypothesis that steroid hormones promote the outgrowth of ER-negative cancers by influencing host cell types distinct from the breast epithelium itself. We demonstrated that increasing the levels of circulating estrogens is sufficient to promote the formation and progression of ER-negative cancers while, pharmacologically inhibiting estrogen synthesis following pregnancy prevents ER-negative tumor formation. Moreover, we demonstrate that the effects of estrogen act via a systemic increase in host angiogenesis, in part through increased mobilization and recruitment of bone marrow stromal derived cells into sites of angiogenesis and to a growing tumor mass. Taken together, these data suggest that estrogen may promote the growth of ER-negative cancers by acting on cells distinct from the cancer cells to stimulate angiogenesis.