Feline Infectious Peritonitis (FIP) is an important viral disease of domestic cats, particularly those in multiple cat environments such as catteries and shelters. FIP-associated mortality is very high and the lack of effective treatment or reliable antemortem tests is a cause of frustration both for the veterinarian and owner. This paper presents a case report of an adult cat with a presumptive diagnosis of FIP, illustrating the difficulty in making a definitive diagnosis of this disease.