Massive hydrothorax complicating continuous ambulatory peritoneal dialysis (CAPD) is relatively rare. A 67- year-old male and a 23-year-old female patients during CAPD presented massive pleural effusion, They have been performing peritoneal dialysis due to end-stage renal disease for 8 months and 2 weeks respectively. We injected 99mTc-labelled radiopharmaceutical (phytate and MAA, respec- tively) into peritoneal cavity witb the dialysate. The anterior, posterior and right lateral images were obtained. The studies reveal visible radioactivity in the right chest indicating the communication between the peritoneal and the pleural space. After sclerotherapy with tetracycline, the same studies reveal no radioactivity in the right chest suggesting successful therapy. We think nuclear imaging is a simple and noninvasive method for the differential diagnosis of pteural effusion in patients during CAPD and the evaluation of therapy.