Occult pneumothorax is defined as a pneumothorax that is detected by abdominal computed tomographic (CT) scanning, but not routine supine screening chest roentgenograms. Forty trauma patients with occult pneumothorax were prospectively randomized to management with tube thoracostomy (n = 19) or observation (n = 21) without regard to the possible need for positive pressure ventilation, to test the hypothesis that tube thoracostomy is unnecessary in this entity. Eight of the 21 patients observed had progression of their pneumothoraces on positive pressure ventilation, with three developing tension pneumothorax. None of the patients with tube thoracostomy suffered major complications as a result of the procedure. Hospital and ICU lengths of stay were not increased by tube thoracostomy. Patients with occult pneumothorax who require positive pressure ventilation should undergo tube thoracostomy.