The present study was designed to test whether there was a significant improvement in pulmonary function and arterial blood oxygenation after therapeutic thoracentesis on patients with inversion of a hemidiaphragm due to pleural effusion. In 21 patients with inversion of a hemidiaphragm because of a pleural effusion, we studied the changes in pulmonary mechanics and gas exchange that occurred in 24 h after removal of 600 to 2,700 mL of fluid by thoracentesis. There was a small but significant increase in the forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC) (p < 0.001). The alveolar-arterial oxygen gradient (P[A-a]O2) and partial pressure of arterial oxygen (PaO2) showed a significant increase (p < 0.001), but there was no change in partial pressure of arterial carbon dioxide (PaCO2). In the present study, all patients with a large pleural effusion had inversion of a hemidiaphragm documented by chest sonography, and that was an important factor to observe significant improvement in pulmonary mechanics and gas exchange.