Hydrothorax is an infrequent complication of peritoneal dialysis. Hydrothorax is often secondary to diaphragmatic defects. Available diagnostic tools are usually insensitive and false-negative results are common. We describe a modification to the standard isotopic peritoneogram that proved effective when other tests yielded false-negative results. We conclude that thoracocentesis prior to the standard isotopic peritoneogram facilitates the diagnosis of diaphragmatic defects.