Injection-site sarcomas are a relatively common type of cancer in cats with an estimated incidence of 1 in 1,000 to 1 in 10,000 of all vaccines administred to cats. As more is understood about these tumors, more is also being discovered about what veterinarians may do to prevent their development. This paper will present one case of an injection-site fibrosarcoma in a cat. In addition, the biologic behavior of injection-site sarcomas, prognostic and risk factors, diagnostic and treatment principles, as well as practical guidelines for veterinarians in the prevention of these tumors, will be discussed.