In most human breast cancers, tumor cell proliferation is estrogen dependent. Although hormone-responsive tumors initially respond to anti-estrogen therapies, most of them eventually develop resistance. Our goal was to identify alternative targets that might be regulated to control breast cancer progression. Sulforhodamine B assay was used to measure the viability of cultured human breast cancer cell lines exposed to various inhibitors. Protein expression in whole-cell extracts was determined by Western blotting. BT-474 tumor xenografts in nude mice were used for in vivo studies of tumor progression. RO 48-8071 ([4'-[6-(Allylmethylamino)hexyloxy]-4-bromo-2'-fluorobenzophenone fumarate]; RO), a small-molecule inhibitor of oxidosqualene cyclase (OSC, a key enzyme in cholesterol biosynthesis), potently reduced breast cancer cell viability. In vitro exposure of estrogen receptor (ER)-positive human breast cancer cells to pharmacological levels of RO or a dose close to the IC50 for OSC (nM) reduced cell viability. Administration of RO to mice with BT-474 tumor xenografts prevented tumor growth, with no apparent toxicity. RO degraded ERα while concomitantly inducing the anti-proliferative protein ERβ. Two other cholesterol-lowering drugs, Fluvastatin and Simvastatin, were less effective in reducing breast cancer cell viability and were found not to induce ERβ. ERβ inhibition or knockdown prevented RO-dependent loss of cell viability. Importantly, RO had no effect on the viability of normal human mammary cells. RO is a potent inhibitor of hormone-dependent human breast cancer cell proliferation. The anti-tumor properties of RO appear to be in part due to an off-target effect that increases the ratio of ERβ/ERα in breast cancer cells.