The purpose of this study was to determine whether changes in pulmonary function after therapeutic thoracentesis are related to the pleural pressure or to changes in pleural pressure during thoracentesis. Spirometry was obtained before and 24 h after thoracentesis in 26 patients. Pleural pressures were measured with a U-shaped manometer initially and after each 400-ml aliquot of pleural fluid was removed. Thoracentesis was continued until the patient developed severe symptoms (chest pain or coughing), the pleural pressure dropped below -20 cm H2O, or no more fluid could be obtained. The mean vital capacity improved 410 +/- 390 ml in this group of patients who had 1,740 +/- 900 ml fluid removed. The improvement in the VC most closely correlated with the pleural pressure after 800 ml fluid had been withdrawn (r = 0.57, p less than 0.005). The ratio of the improvement in the VC to the amount of fluid removed most closely correlated with the pressure change after 800 ml fluid had been removed (r = -0.43, p less than 0.05). From this study we conclude that the improvement in the FVC after therapeutic thoracentesis is small relative to the amount of fluid withdrawn. Patients with higher pleural pressures after the removal of 800 ml pleural fluid and patients with smaller decreases in the pleural pressure after removal of 800 ml pleural fluid have greater improvements in their pulmonary functions after thoracentesis.