Feline gastrointestinal eosinophilic sclerosing fibroplasia (FGESF) is a disease in cats, which is associated with masses in the gastrointestinal tract and less often in the adjacent mesentery or mesenteric lymph nodes. Cats present with clinical signs including vomiting, decreased appetite, weight loss, and lethargy. Recommended treatment is the surgical removal of the mass, which causes clinical remission in only some cases. Why or how these masses develop is unknown. We have collected 34 cases of FGESF from across the world. Our overall goal is to determine the presentation, clinical findings, ultrasonographic findings, and microscopic findings associated with FGESF. Preliminary data for this grant include clinical findings and microscopic features. Although FGESF has been speculated to be within the spectrum of mast cell tumors, our findings indicate mast cells to be rare in the tissues stained with hematoxylin and eosin. We specifically aim to investigate two potential causes for this disease: inflammatory response to parasites (Toxocara cati) or mast cell tumor. We hypothesize that mast cell numbers will remain low in IHC stained slides and that T. cati DNA will be identified in some of the cases. The results of this study will have a profound impact on understanding the pathogenesis of FGESF and potentially lead to increased use of anthelminthics to decrease the prevalence of this disease.