The increasing use of CT for the evaluation of blunt abdominal trauma has diagnosed undetected pneumothoraces in many patients. We performed a retrospective study at a major trauma center to determine the incidence of occult pneumothorax in the trauma patient. All trauma patients (3121) admitted to a Level I trauma center over a 51-month period were reviewed to determine the incidence of pneumothorax and occult pneumothorax, the method of diagnosis, and treatment. All major trauma patients received a chest X-ray (CXR), whereas 842 patients (27%) underwent an abdominal CT scan. In the 172 patients diagnosed with pneumothorax, 157 abdominal CT scans were performed and 143 were positive for pneumothorax. CXR revealed 49 right-sided pneumothoraces and 58 left-sided pneumothoraces in 95 patients. Abdominal CT scans diagnosed 73 right pneumothoraces and 90 left pneumothoraces in 143 patients. There were 67 patients (71 thoraces) who were seen to have a pneumothorax on abdominal CT scan not seen on admission CXR. This represents 2.2 per cent of all trauma patients and 7.9 per cent of patients with abdominal CT scans. In looking at just trauma patients with pneumothorax, the CT scan was responsible for diagnosing 39 per cent of the patients with a pneumothorax. The occult pneumothorax is being diagnosed more frequently as methods of evaluating and diagnosing trauma patients become more sensitive.