Abstract Breast cancer remains the most common cancer and is the second leading cause of cancer death in women. The magnitude of this problem provides strong rationale for studies that may lead to the development of new chemopreventative strategies and/or lifestyle changes that reduce cancer incidence. It is of significance, therefore, that circulating cholesterol has been correlated to the development of breast cancer, and patients taking lipophilic inhibitors of 2-hydroxy-3-methylglutaryl coenzyme A (statins) demonstrate significantly lower breast cancer incidence, and decreased breast cancer recurrence. Therefore, our recent finding that 27-Hydroxycholesterol (27HC), a primary metabolite of cholesterol, exerts partial agonist activity on both the estrogen receptors (ERs) and liver X receptors (LXRs) suggests a potential, mechanistic link between hypercholesterolemia and breast cancer incidence. Importantly, the enzyme responsible for the conversion of cholesterol to 27HC (CYP27A1) is primarily expressed in macrophages, and it is well established that tumor infiltrating macrophages are associated with more aggressive tumors and worse patient outcome. Therefore, we initiated a series of studies to determine the impact of 27HC on breast cancer pathophysiology. Here, we demonstrate that (1) macrophage produced 27HC increases breast cancer cell proliferation, (2) elevation of exogenous 27HC increases in vivo tumor growth and lung metastases, and (3) genetic ablation of the enzyme responsible for 27HC synthesis increases latency, and reduces tumor growth and metastasis. Our ongoing work is aimed at identifying the mechanisms responsible for the 27HC induced increase in metastasis. In summary, our data thus far strongly indicate a role for macrophage produced 27HC in breast cancer pathophysiology and metastasis. The results provide additional support for the exploration of potential chemopreventative benefits of lower cholesterol diets, pharmacological inhibitors of HMG-CoA reductase or CYP27A1, and macrophage ablative strategies. DOD Postdoctoral Award BC085585 (ERN), NIH 5R37DK048807 (DPM). This abstract is also presented as Poster B50. Citation Format: Erik Russell Nelson, Suzanne E. Wardell, Matthew K. Howe, Nicole J. Carver, Michihisa Umetani, Donald P. McDonnell. The cholesterol metabolite 27-hydroxycholesterol increases breast cancer tumor growth and metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr PR3.