Body effusions from 197 cats and blood serum samples from 252 cats, where Feline Infectious Peritonitis (FIP) was part of the differential diagnosis, were analysed. The diagnoses were confirmed by clinical follow up or histopathology. The final diagnosis FIP was always confirmed by histopathology. The median age of cats with FIP was 1.6 years. FIP was responsible for 41% of the body effusions, whereas malignomas caused 24%, cardial insufficiencies 14% and purulent serositis 12% of the body effusions. The rivalta test was highly sensitive for FIP. Predictive value of a negative result was 100%, predictive value of a positive result was 84%. In half of the cases with purulent serositis and in 20% of malignomas rivalta reacted positive. The cardial insufficiencies were negative for rivalta. Coronavirus antigen could be demonstrated by immunofluorescence in 34 of 49 body effusions caused by FIP, whereas in the 50 body effusions caused by other diseases no coronavirus antigen was detected. An albumin globulin ratio of < 0.6 was highly diagnostic for an inflammatory process, nearly exclusively for FIP. An albumin globulin ratio of > or = 0.8 almost excluded FIP. Only a negative or very high (1:1600) FIP titer could contribute to confirm diagnosis. Low and medium titers, however, should not be interpreted.