BACKGROUND: Measurement of pleural fluid constituents are of value in the diagnosis of pleural effusions and in the seperation of exudates from transudates. The position of the patient (sitting or lying) prior to thoracentesis may result in difference in the measurement of these constituents. The purpose of this study is to determine whether postural differences in pleural fluid constituents exist, and if so, whether they are of any clinical significance. METHOD: 41 patients with pleural effusions on chest roentgenography were prospectively studied. The fluid cell counts, partial gas tension, and concentrations of chemical constituents were compared in the supine and upright positions. RESULTS: 1) A total of 10 patients were found to have an transudative effusion. In the transudates there was no significant difference in pleural fluid constituents according to posture change. 2) A total of 31 patients were found to have an exudative effusion. Statistically significant postural changes were noted in pH, WBC counts, protein, and LDH concentrations in the exudates. It may be due to postural sedimentary effect in the pleural space. 3) The PC02 measurements and glucose concentration were not affected by changes in position in exudates or transudates. CONCLUSION: Postural sedimentary effect occurs in the pleural space with reference to the measurement of certain pleural fluid constituents when an inflammatory process is present. Therefore it is recommended that thoracentesis after 30 minutes in the sitting position should be performed.