A prospective analysis of supine radiographs in 40 patients with pleural effusions was undertaken to determine the radiographic manifestations of pleural effusion on supine radiographs and the quantity of fluid that may be detected in the supine position. The presence of pleural effusion was predicted from supine radiographs using the following signs: increased homogeneous density superimposed over the lung fields, obliteration of the silhouette of the diaphragm, meniscus sign, apical capping, and accentuation of the right minor fissure. The supine radiographic findings were correlated with findings on subsequent standing erect posteroanterior and lateral radiographs. This study indicates that effusion of sufficient quantity to produce blunting of the lateral costophrenic angle on an erect posteroanterior radiograph (175-525 ml) will produce a noticeable increase in the density of the lower lung zone on supine radiographs. As the quantity of effusion increases, the abnormality of the supine chest radiograph increases in a stepwise manner. The classical findings of increased density over the entire hemithorax and apical capping described in pleural effusion on supine radiographs are late findings seen typically in large effusions.