Although a number of radiologic signs of pneumothorax in the supine patient have been reported, the frequency of involvement of various pleural recesses has not been emphasized. In 88 critically ill patients with 112 pneumothoraces, the anteromedial (38%) and subpulmonic (26%) recesses were the most commonly involved in the supine and semirecumbent position. In this study, 30% of pneumothoraces were not initially detected by the clinician or radiologist, and half of these progressed to tension pneumothorax. Knowledge of the most common recesses involved in pneumothorax and aggressive use of additional radiographic views, including computed tomography, should increase detection of pneumothoraces in critically ill patients.