The records of 92 patients with flail chest injury treated at a Level I trauma center were analyzed retrospectively. Associated intrathoracic injuries included pulmonary contusion (46 percent) and pneumothorax or hemothorax, or both (70 percent). The incidence of great vessel, tracheobronchial and diaphragmatic injuries was no different from that of a control population with simple rib fractures. Adult respiratory distress syndrome developed in 27 percent of patients with flail chest; 69 percent of all patients required ventilation (mean duration, 22 days). Mean length of hospital stay was 24 days. The mortality rate was 33 percent. We conclude that flail chest serves as a marker of significant intrathoracic injury, highly associated with pulmonary contusion, but even more so with pneumothorax or hemothorax. Flail chest does not seem to be a marker for great vessel, tracheobronchial, or diaphragmatic injuries. The majority of patients (more than two-thirds) will require mechanical ventilation for prolonged periods. Of paramount importance is the recognition of flail chest as a marker of high kinetic energy absorption, resulting in life-threatening thoracic as well as nonthoracic injuries.