Of 457 patients with multisystem injuries undergoing abdominal computed tomographic (CT) scan, 26 patients were found to have 31 pneumothoraces. None of these were apparent on prior roentgenograms of the chest. Each pneumothorax was quantified by measuring its maximal width in millimeters and the number of 10 millimeter CT sections on which it appeared. Serial roentgenograms of the chest and patient charts were reviewed. The major factor determining the clinical course and management of these pneumothoraces was size. Seventeen per cent of pneumothoraces measuring less than 5 X 80 millimeters (group 1) and 85 per cent of those measuring greater than or equal to 5 X 80 millimeters (group 2) had tube thoracostomy performed. The percentage of pneumothoraces in each group with positive pressure ventilation was 55 and 77 per cent, respectively. Our results suggest that such occult pneumothoraces may be managed with close observation if they measure less than 5 X 80 millimeters, whether or not the patient is to receive positive pressure ventilation. Larger pneumothoraces and those associated with more than two rib fractures may require early treatment.