A prospective analysis of anteroposterior supine radiographs in 34 patients was undertaken to determine the detectability of pleural effusions on supine radiographs. The presence of pleural effusions and quantity of fluid (small, moderate, or large) were evaluated by using the following radiographic signs: increased homogeneous density superimposed over the lung, loss of the hemidiaphragm silhouette, blunted costophrenic angle, apical capping, elevation of the hemidiaphragm, decreased visibility of lower-lobe vasculature, and accentuation of the minor fissure. Decubitus radiographs were performed to identify and to estimate the quantity of pleural fluid. Sixty-two hemithoraces were evaluated by three observers. From a total of 36 pleural effusions shown on decubitus views, 24 were correctly identified on supine radiographs (sensitivity of 67%, specificity of 70%, and accuracy of 67%). The most frequent but least specific criterion for detecting pleural effusions on supine radiographs is blunting of the costophrenic angle. Other helpful signs include loss of the hemidiaphragm and increased density of the hemithorax. A normal supine radiograph does not exclude a pleural effusion. Our results show that supine radiographs are only moderately sensitive and specific for the evaluation of pleural effusions.