The demonstration of small amounts of pleural effusion has been a difficult problem. The typical “S” line of Ellis is now an accepted finding in physical diagnosis and in routine chest x-ray projections. It is also a well established fact that routine roentgenograms may fail to reveal as much as 500 ml. of pleural fluid. This occult fluid may well be concealed in the infrapulmonary space, the portion of the pleural cavity between the diaphragm and the inferior surface of the lung. Rigler (6) was probably the first radiologist to employ special positions for the diagnosis of small pleural effusions. By use of the lateral decubitus position, with a horizontal x-ray beam he was able to show minimal quantities of fluid. He reported 5 cases in which fluid was demonstrated in this position, while the upright postero-anterior projection presented a normal appearance. In 3 other cases, lateral decubitus films established a diagnosis of pleural effusion which had been equivocal on routine views. Between 1931 and 1...