Acute massive right-sided hydrothorax is a relatively rare but serious complication of peritoneal dialysis and may be responsible for the development of dyspnea during peritoneal dialysis. The pleural fluid appears to arise from the peritoneal dialyzate based on the time of its appearance and its chemical composition. It should be included in the differential diagnosis when a patient becomes dyspneic during peritoneal dialysis along with exaccerbation of congestive heart failure, pneumonia, atelectasis, and purulent bronchitis. Its occurrence is an indication to stop the peritoneal dialysis and contraindicates further use of this form of dialysis. Treatment may be conservative or aggressive (thoracentesis) depending on the clinical condition of the patient. Etiology is poorly understood. In some cases, there may be traumatic diaphragmatic fenestrations, but the majority of cases appear to be due to less well defined communications between the peritoneal and pleural spaces.