Expression of the mouse mammary tumor virus (MMTV) neu/erbB2 transgene in mice induces mammary tumors. To examine the effect of removing estrogen receptor α (ERα) signaling on the ability of an MMTV-neu/erbB2 transgene to induce mammary tumors, the neu transgene was expressed in the ERα knockout (αERKO) mouse, which lacks functional ERα. MMTV-neu females that lacked ERα still developed mammary tumors; however, tumor onset was significantly delayed. This study indicates that ERα is not required for mammary tumor induction by overexpression of neu/erbB2, but plays a role in the rate of tumor onset. The removal of ovarian steroid by ovariectomy in adults did not alter the onset rate. In contrast, prepubertal ovariectomy, which arrested mammary epithelial development, significantly delayed onset. In addition, manipulations that increase progesterone also accelerate the tumor onset, indicating the slower onset in the αERKO is primarily attributable to the anovulatory phenotype resulting in lack of progesterone stimulation and a decreased abundance of target cells in the αERKO mammary gland.