This report describes a case involving an 8-year-old domestic short-haired cat that had a palpable mass in the abdomen as an incidental finding on physical examination. Fine needle aspiration of the mass identified it as a sarcoma. The mass was surgically removed, and when cut for microscopic examination, a surgical sponge remaining from a previous surgery was found. Microscopic examination revealed the mass to be a fibrosarcoma arising at the site of the surgical sponge. The sponge is believed to have stimulated inflammation, leading to formation of a granuloma. While transformation of a granuloma into a cancerous growth at the site of a retained surgical sponge has been documented in dogs, this is the first report of this type of granuloma developing into a fibrosarcoma in a cat. [MK]