Introduction: Breast cancer is one of the most common malignancies affecting women in the United States and Europe. Approximately three out of every four women with breast cancer develop metastases in bone which, in turn, diminishes quality of life. The αvβ3 integrin has previously been implicated in multiple aspects of tumor progression, metastasis and osteoclast bone resorption. Therefore, we hypothesized that the αvβ3-selective inhibitor, S247, would decrease the development of osteolytic breast cancer metastases. Materials and methods: Cells were treated in vitro with S247 and assessed for viability and adhesion to matrix components. Athymic mice received intracardiac (left ventricle) injections of human MDA-MB-435 breast carcinoma cells expressing enhanced green-fluorescent protein. Mice were treated with vehicle (saline) or S247 (1, 10, or 100 mg/kg/d) using osmotic pumps beginning either one week before or one week after tumor cell inoculation. Bones were removed and examined by fluorescence microscopy and histology. The location and size of metastases were recorded. Results and conclusions: IC50 for S247 adhesion to αvβ3 or αIIBβ 3a substrates was 0.2 nM vs. 244 nM, respectively. Likewise, S247 was not toxic at doses up to 1000 μM. However, osteoclast cultures treated with S247 exhibited marked morphological changes and impaired formation of the actin sealing zone. When S247 was administered prior to tumor cells, there was a significant, dose-dependent reduction (25-50% of vehicle-only-treated mice; P = 0.002) in osseous metastasis. Mice receiving S247 after tumor cell inoculation also developed fewer bone metastases, but the difference was not statistically significant. These data suggest that, in the MDA-MB-435 model, the αvβ3 integrin plays an important role in early events (e.g., arrest of tumor cells) in bone metastasis. Furthermore, the data suggest that αvβ3 inhibitors may be useful in the treatment and/or prevention of breast cancer metastases in bone.