This report describes an intestinal adenocarcinoma in a 13-year-old female lion ( Panthera leo) from the zoo of Bursa, Turkey. The animal suffered from muscle atrophy, was presented to the Department of Internal Medicine with signs of anorexia, lethargy and abdominal distension, and a presumptive diagnosis of FIP was made. The lion did not respond to treatment and was euthanized for humane reasons. At necropsy, approximately 10 l serous ascites was observed in the abdominal cavity. A firm tumourous mass with grayish white cut surface was seen to invade all layers of the wall of the ascending colon. The tumour did not cause any luminal stricture, and no mucosal ulcerations were detected. In addition, numerous firm, grayish white nodules up to 1.5 cm in diameter were present on the peritoneal surface of the abdominal wall, omentum, diaphragm, mesentery and also on the serosal surfaces of stomach, spleen, uterus and vagina. Microscopically, a metastasizing mucinous type of a colonic adenocarcinoma with metaplastic bone formation was found. Peritoneal implants had similar features to the intestinal tumour except for the lack of metaplasic bone formations. Immunohistochemically, neoplastic cells were positive for CDX2, a marker for a human intestinal transcription factor in normal and neoplastic intestinal epithelial cells. However, cytokeratins 7 and 20, usually used in human diagnostics of intestinal adenocarcinomas, were negative, suggestive of an intestinal epithelial cell origin. Depending on the gross, histopathologic and immunohistochemical findings, the tumour was diagnosed as mucinous-type colonic adenocarcinoma. This is the first report of intestinal adenocarcinoma in lions. [ABSTRACT FROM AUTHOR]