Recognizing pneumothorax and hydropneumothorax on chest x-rays of supine ICU patients requires attention to areas other than the lung apex. Pleural air in a supine patient collects in the anterior costophrenic sulcus, producing hyperlucency over the upper abdominal quadrants, and the deep costophrenic sulcus sign. Hydropneumothorax may be recognized when a sharp pleural line is bordered by increased opacity within the pleural space. The presence of pneumothorax and hydropneumothorax can be confirmed by decubitus or upright chest x-rays.