2 patients on continuous ambulatory peritoneal dialysis (CAPD) developed massive hydrothorax. Further studies revealed preferential transfer of fluid from peritoneal cavity to pleural space in absence of a direct pleuroperitoneal communication. Hydrothorax resolved in both following a brief interruption in CAPD but recurred in one upon restitution of CAPD. In case of recurrent hydrothorax, pleurodesis or alternate dialytic therapy should be considered.