Canine osteosarcoma (OSA) is an insidious disease accounting for the majority of skeletal malignancies. Recently, examination of dual cyclooxygenase and 5-lipoxygenase inhibitors in human and canine oncology suggests that 5-lipoxygenase inhibitors may be an effective approach for prevention and/or treatment. We examined 5-lipoxygenase expression in primary canine osteosarcoma samples and have shown that approximately 65% of osteosarcoma’s express cytoplasmic 5-lipoxygenase. Further examination of a cell culture and a xenograft model shows similar 5-lipoxygenase expression. Surprisingly, a canine 5-lipoxygenase inhibitor (tepoxalin) significantly reduced cell proliferation at physiologic doses in-vitro and diminished xenograft tumor growth in nude mice suggesting therapeutic utility of this inhibitor. The lack of nuclear staining in primary and xenografted tumors, and lack of response to lipid mediator re-introduction, suggests that lipid mediator production is not the primary means by which tepoxalin acts to alter proliferation. Regardless of the mechanisms involved in retarding cell proliferation, tepoxalin might be a viable treatment option for canine osteosarcoma which warrants further clinical investigation.