Superior mediastinal widening, as seen on chest radiographs of traumatized patients, has been considered the hallmark of mediastinal injury. The usual causes of superior mediastinal widening are rupture of the aorta, esophagus or trachea, and hematoma as a result of spinal fracture. Posterior dislocation of the sternoclavicular joint is rarely a cause. We report a case of superior mediastinal widening resulting from traumatic posterior dislocation of the stern-oclavicular joint, and describe the CT findings, including those of 3-D imaging.