Two cases of right hepatic hydrothorax occurring in the absence of clinical ascites are reported. Diagnosis was confirmed by the intraperitoneal and intrapleural injection of radioisotope 99mTc-sulfur colloid that demonstrated the one-way transdiaphragmatic flow of fluid from the peritoneal to pleural cavities. In contrast, radioisotope injected into the peritoneal cavity of 5 patients with pleural effusions secondary to pulmonary or cardiac disease failed to traverse the diaphragm and localize in the pleural space. Medical therapy with salt and water restriction and diuretics resulted in both of the patients with hepatic hydrothorax developing signs of intravascular volume depletion without significant mobilization of the pleural fluid. Thoracotomy allowed identification of the diaphragmatic defects that were repaired by chemical and traumatic pleurodesis followed by postoperative peritoneal and pleural drainage. This therapy resulted in complete resolution of the pleural effusions, which have not recurred despite the subsequent development of clinical ascites in both patients.