Hydrothorax was identified in a 14-year-old Siamese cat with a pre-exi
stent perinephric pseudocyst. The pleural fluid was classified as a lo
w-protein transudate. Intrapseudocystic scintigraphy confirmed a direc
t communication between the pseudocyst and the pleural space, The hydr
othorax resolved following pseudocystectomy and unilateral nephrectomy
, demonstrating that the pseudocyst caused the hydrothorax.