We used high-resolution ultrasonography to image the ulnar collateral ligament in 39 patients who had sustained recent injuries of the metacarpophalangeal joint of the thumb. All the patients were subsequently operated on and the lesions of the ligament were recorded. In 36 patients the preoperative ultrasonographic diagnosis was correct. Five of these showed no rupture of the ligament. In the other 31, ultrasonography correctly distinguished between rupture in situ (15) and rupture with dislocation of the ligament (16). Misdiagnosis by ultrasonography in three cases was due to delay of the investigation (three weeks after injury) in one, to technical error in one and to misinterpretation of the image in one.