Massive hydrothorax occurring during continuous ambulatory peritoneal dialysis is a rare complication that may appear at any time during the course of the treatment. Most of the time, it is right-sided and is more frequent in female patients. A case is presented that was investigated by peritoneopleural scintigraphy. A rapid accumulation of the radiopharmaceutical over the right hemithorax was observed. To explain that finding, a macroscopic diaphragmatic defect was suspected. Different therapeutic approaches are presented.