It is difficult to explain why fluid may sometimes collect between the under surface of the lung and the diaphragm without spreading freely in the greater pleural space. It is hoped that the presentation and discussion of some examples of this situation will be of assistance in the better understanding of this phenomenon. Others have observed and recorded this occurrence and have listed the factors which they regard as important in the mechanics of the process. Rigler (1) described clearly some of the forces which modify the effects of gravity on fluid accumulation in the pleural space. According to him, these modifying factors are: (1) retractility of the lung, which may in turn be altered by the presence of consolidation, fibrosis, cavitation, emphysema, or atelectasis; (2) a tendency to cohesion between pleural surfaces, producing an effect of capillarity on the position of the fluid; (3) surface tension and viscosity of the fluid, both of which may be related to its protein and lipoid content; (4) presence of gas in the pleural cavity, tending to neutralize the effects of the other three factors and completely eliminating the force of capillarity. Lipschultz (2) reported a case of this type with a lymphoblastoma as the underlying lesion. In his conclusions he mentions essentially the same factors as did Rigler as being responsible for the atypical roentgenographic appearance of the fluid. Sante (3) has discussed this problem as it is related to subphrenic disease. He states that basal accumulations of fluid frequently occur as a result of subdiaphragmatic inflammatory lesions. Grier (4) has reported on pyothorax localized in the basal area by adhesions forming a closed space in that region. Yater and Rodis (5) also reported on fluid accumulation simulating elevation of the diaphragm. Case Reports Case 1: J. W. gave a four-year history of intermittent pleurisy in the left side of the chest. On his admission to the hospital the findings were typical of nasopharyngitis and the chest roentgenogram was entirely negative. One week later he had left lower chest pain and the temperature rose to 100°. A chest roentgenogram now showed what was thought to be an elevation of the left hemidiaphragm. On subsequent films it was noted that a wide zone of density separated the stomach bubble from the base of the lung and a diagnosis of left basal effusion was made. This was confirmed by a left thoracentesis, which yielded a transudate type of fluid. The situation is illustrated in Figure 1, made from another case. Following the thoracentesis, the effusion assumed the usual appearance of intrapleural fluid as seen on the posterior-anterior roentgenogram. The illusion of elevated left hemidiaphragm no longer existed. Comment: This case illustrates two important features of effusions of this type. First, it demonstrates the value of the stomach bubble as a diagnostic aid when the process is on the left.