A 58 year old man was admitted to the accident and emergency department following an industrial accident in which he sustained a three part fracture dislocation of his right humerus. Chest radiography revealed a large mass in the right upper hemithorax and, when the patient became hypotensive, an emergency thoracotomy was performed. The mass was found to be a massive intrathoracic lipoma. This case shows how preexisting intrathoracic lesions may be mistaken for subclavian or great vessel trauma following violent shoulder girdle injury. The differential diagnosis of traumatic and non-traumatic intrathoracic mass lesions in chest radiography should be considered carefully.