Abstract We previously observed that chronic consumption of a high fat-diet (HFD) increased the breast cancer-related mortality in the 4T1 orthotopic mouse breast cancer model. In the present study, we determined whether feeding mice with a HFD for a prolonged period increases solid tumor growth and metastasis of breast cancer cells using the 4T1 orthotopic model. 4-week old, female BALB/c mice were fed a purified diet containing 60 kcal% fat (HFD) or 10 kcal% fat (control diet) for a period of 16 weeks. After 16 weeks, 4T1 mammary carcinoma cells (5 × 104 cells) were injected into the inguinal mammary fat pad of syngeneic female BALB/c mice and the mice were continuously fed the same diets. Solid tumor growth was increased in the HFD group as compared to the control diet group. Additionally, chronic consumption of the HFD markedly increased the number and volume of tumor nodules in the lung and liver. The expression of cyclin-dependent kinase (CDK)2, CDK4, cyclin D1, cyclin A, Ki67, vascular endothelial growth factor, CD31 (an angiogenesis marker), and CD45 (a monocyte/macrophage marker) was markedly increased in the tumor tissues in the HFD group as compared to control diet group. The protein levels of urokinase-type plasminogen activator (uPA), intercellular adhesion molecule (ICAM)-1, and vascular cell adhesion molecule-1 were significantly increased but those of plasminogen activator inhibitor-1 were decreased in the lung tissues of the HFD group as compared to control diet group. The serum levels of complement fragment 5a (C5a), interleukin (IL)-6, macrophage colony stimulating factor (M-CSF), soluble intercellular adhesion molecule (sICAM)-1, tissue inhibitors of metalloproteinase (TIMP)-1, and triggering receptor expressed on myeloid cells (TREM)-1 were up-regulated in the HFD group as compared to control diet group. We conducted in vitro assays to determine effect of these cytokines, which had been increased in the sera of mice fed on the HFD, on the cell proliferation, adhesion, and migration of 4T1 cells. sICAM-1 increased the viability of 4T1 cells. TREM-1, M-CSF, and sICAM-1 significantly increased the migration of 4T1 cells. Additionally, the capacity of 4T1 cells to adhere to strips coated with human collagen type I was increased by C5a, sICAM-1, IL-16, M-CSF, TIMP-1, and TREM-1. These results indicate that, in addition to adhesion molecules, metalloproteases and pro-angiogenic factors, the cytokines C5a, sICAM-1, IL-16, M-CSF, TIMP-1, and TREM-1 play important roles in the metastasis of mammary cancer cells in mice fed on a HFD for a prolonged period. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5562. doi:10.1158/1538-7445.AM2011-5562