Obesity and altered lipid metabolism are risk factors for breast cancer in pre- and post-menopausal women. These pathologic relationships have been attributed in part to the impact of cholesterol on the biophysical properties of cell membranes and to the influence of these changes on signaling events initiated at the membrane. However, more recent studies have indicated that the oxysterol 27-hydroxycholesterol (27HC), and not cholesterol per se, may be the primary biochemical link between lipid metabolism and cancer. The enzyme responsible for production of 27HC from cholesterol, CYP27A1, is expressed primarily in the liver and in macrophages. In addition, significantly elevated expression of this enzyme within breast tumors has also been observed. It is believed that 27HC, acting through the liver X receptor in macrophages and possibly other cells, is involved in maintaining organismal cholesterol homeostasis. It has also been shown recently that 27HC is an estrogen receptor agonist in breast cancer cells and that it stimulates the growth and metastasis of tumors in several models of breast cancer. These findings provide the rationale for the clinical evaluation of pharmaceutical approaches that interfere with cholesterol/27HC synthesis as a means to mitigate the impact of cholesterol on breast cancer pathogenesis. Cancer Res; 74(18); 4976-82. ©2014 AACR.