Feline infectious peritonitis is an immunologically mediated vasculitis and perivasculitis, with characteristic inflammatory lesions oriented around blood vessels in multiple organs. The causative organism is a coronavirus which is closely related to the Feline Enteric Coronaviruses, to Transmissible Gastroenteritis Virus of swine, to Canine Coronavirus, and to Human Bronchitis Virus. FIP is a disease affecting all members of the cat family. It has been reported in the lion, cheetah, mountain lion, jaguar, black leopard, and in several species of smaller exotic cats. It is worldwide in occurrence. Clinical FIP is a fairly common disease. The disease was diagnosed in less than one percent of the cats presented to the University of California Veterinary Teaching Hospital. Even in catteries experiencing problems with FIP, the morbidity is rarely greater thatn 10 percent and most often much lower. FIP occurs most commonly in cats four years of age or less. It is a young cat disease. At the present time FIP should be considered a fatal disease. A cat presented with FIP probably has 6 to 12 weeks or less to live. No satisfactory treatment exists.