An 8-year old castrated domestic long-hair cat was presented with a two week history of abdominal distension. Physical examination revealed a non-painful, fluctuant, palpable mass in the right craniodorsal abdomen, and unilaterally muffled heart sounds on the right thorax. Routine clinico-pathological values were unremarkable apart from mild azotemia with a concurrent urine specific gravity of 1.031, which reflect a degree of renal dysfunction. Radiographic and ultrasound examinations of the thorax revealed the cardiac enlargement to be due to the congenital peritoneo-pericardial diaphragmatic hernia with liver occupying the right half of the pericardial sac. There was also a mild gypertrophy of the heart. Radiography and ultrasonography of the abdomen showed the mass to be composed of a large fluid filled cystic structures surrounding the right and left kidneys, and the kidneys themselves were of increased echogenecity. A diagnosis of perinephric pseudocysts was made. The patient responded well to the surgical procedures. Perinephric pseudocysts and peritoneo-pericardial diaphragmatic hernia in the cat are rare, and a case is described and the literature is reviewed in this report.