In a significant percentage of the population, a previously undescribed fracture fragment of the supinator crest of the ulna could be detected. The most frequently occurring associated injuries were fractures of the radial head, the coronoid process, and the distal humerus. The aetiology of these lesions is unknown; however, bony avulsion of the annular or the lateral ulnar collateral ligament seems to be the most likely cause. If this fragment is to be diagnosed by CT, the possibility of lateral or posterolateral instability should be considered.