In 52 patients with pleural effusions, pleural pressures were measured initially and serially as pleural fluid was withdrawn. Pleural fluid aspiration was continued until the pleural pressure fell below -20 cmH2O, or the patient developed excessive symptoms, or no more fluid could be obtained. The initial pleural pressure ranged from +8 to -21 cmH2O. The rate of pleural pressure change as fluid was withdrawn was highly variable. In 13 of 52 procedures (25%), thoracentesis was terminated because the pressure fell below -20 cmH2O. Negative initial pleural pressures and/or rapid changes in the pressures as fluid was withdrawn were suggestive of malignancy or trapped lung. The measurement of pleural pressures in patients with pleural effusions may be useful diagnostically. More importantly, because large changes in pleural pressures are not readily detectable by the operator, pleural pressures should be monitored when large amounts (> 1,000 ml) of pleural fluid are removed to increase the safety of the procedure.