A photostimulable storage phosphor (PSP) computed radiography imaging system was analyzed for its potential to mask pleural effusion during normal image processing. This phenomenon has been observed in several clinical cases in our hospital. To better understand the relationship between pleural effusion and the PSP radiograph appearance, portable radiographs of an athropomorphic chest phantom were acquired with the PSP system in conditions simulating various quantities and distributions of pleural fluid. It was observed that the optical density of the film in one hemithorax was significantly influenced by whether or not fluid was present in the opposite hemithorax. This optical density dependence was determined to be a system-induced effect that results from the image processing (histogram analysis) technique used by the PSP system during image plate readout. It is important to recognize that the PSP system's normal optical density (sensitivity) adjustment can obscure the presence of bilateral pleural fluid accumulation, particularly if the opposite hemithorax contains fluid in an equal or greater amount.