Abdominal CT, which routinely includes the lower thorax, is an important adjunct to supine chest radiography in detecting chest injury in patients with blunt abdominal trauma. In 1,086 consecutive patients with blunt abdominal trauma, 223 of whom had both supine chest radiography and abdominal CT, 49 patients examined with both techniques had pneumothoraces, 28 of them occult (seen only on CT). To help guide management, we established three categories of occult pneumothorax, based on size and location: (a) minuscule (< 1 cm in greatest thickness, seen on four or fewer images); (b) anterior (> 1 cm in greatest thickness, but not extending beyond the midcoronal line); (c) anterolateral (extending beyond the midcoronal line). In our study four of six patients with minuscule pneumothorax, including one who required mechanical ventilation, were observed without complications; two of six patients had chest tube placement. Seven of 14 cases with anterior pneumothorax were observed and resolved without complication; seven had chest tube placement. All eight patients with anterolateral pneumothoraces underwent percutaneous tube thoracostomy, regardless of proposed management.