Feline infectious peritonitis (FIP) is a widespread viral disease of cats and other Felidae. Diagnosis of the disease is difficult and relies on medical history, observed clinical signs, laboratory, molecular and imaging test results, and analysis of the fluid collected. The aim of the study was to perform an ultrasound examination to assess the internal organs of 127 cats with a molecularly confirmed effusive form of feline infectious peritonitis. Of all the animals qualified for the study, 13 cats (10%) had fluid accumulating in the pleural cavity, 26 (21%) individuals had fluid in both the pleural and the abdominal cavities, and 88 cats (69%) only had fluid in the abdominal cavity. The ultrasound examination revealed lesions in the organs, most frequently in the spleen, followed by the liver, gastrointestinal tract, lymph nodes, pancreas, kidneys and gallbladder. An ultrasound examination can significantly facilitate the diagnosis of FIP due to the development of ultrasound technology, improved image resolution, and a greater capacity to recognise small details in the internal organs. The limitation of the examination is, undoubtedly, the subjective analysis of images. It should be noted that further diagnosis of the disease following an ultrasound examination is advisable and that, based on an ultrasound image, it is not possible to diagnose FIP but only suspect it.