A large mesenteric tumour in a Rusa hind (Cervus timoriensis) was diagnosed as a leiomyosarcoma on the basis of tumour size, degree of necrosis, cellular pleomorphism, anaplasia, mitotic activity and evidence of local invasion. Rupture of the tumour and consequent haemorrhage was the immediate cause of death. Both kidneys were enclosed in bony masses which appeared to have replaced the normal perirenal fat. The bone was normal cancellous bone with active haemopoietic marrow. There did not appear to be any connection between the tumour and the metaplastic perirenal bone.