Expeditious surgical management in serious chest trauma improves survival rates. However, guidelines for emergency thoracotomy that depend on the initial amount of blood loss or continuing blood loss are imprecise and frequently require a period of observation. In an attempt to improve prethoracotomy diagnosis emergency thoracoscopy was used in the operating room in 36 patients who presented with hemothoraces. A diagnostic thoracoscope was inserted through the previously placed incisions for chest tubes. The procedure was well tolerated under local anesthesia and in most patients took less than 15 minutes to complete. No complications resulted from thoracoscopy. When the information obtained did not indicate further operations, the thoracoscope was withdrawn and chest tubes were reinserted through the same incisions. The method provided anatomic definition of the injuries in 35 of 36 patients and allowed the determination of whether blood loss was continuing. Management was altered as a result of thoracoscopy in 16 patients (44.4%). An unnecessary thoracotomy was avoided in 16 patients who had greater than 1,500 ml of blood in the thoracic cavity on admission and allowed a more rapid thoracotomy in one patient. Four patients had wounds located close to the diaphragm and laparotomy was avoided when the diaphragm was found free of injury. Bleeding from lacerated intercostal vessels was stopped with diathermy during thoracoscopy in two of three patients. In addition, 15 patients had 200 ml or more of clotted blood removed. The usefulness of removal of clotted blood remains to be proven but it is hoped that the incidence of empyema will be reduced. Emergency thoracoscopy has proven a valuable diagnostic and therapeutic measure in patients sustaining penetrating chest trauma.