1. Six cases of subluxation of the distal radio-ulnar joint by a probable hyperpronation injury are described. In five of the six patients, there is an unusual type of trauma producing the injury; and, in the sixth patient, the presence of severe neuromuscular disease is noted.
2. The injury is a rotatory subluxation of the distal end of the radius on the ulna without evidence of rupture of the dorsal and volar radiocarpal ligaments or the triangular cartilage.
3. The characteristic roentgenographic sign is apparent shifting of the ulnar styloid process laterally when roentgenograms are made in pronation.
4. Possible errors in the making of the roentgenograms at the time of injury are described.
5. Follow-up examination of an untreated injury showed definite disability in the wrist one and a half years later with some limitations of motion.
6. Treatment by closed reduction and immobilization in supination is recommended.
7. An additional case is presented of a three-year-old child who showed all the clinical findings but, because of the lack of ossification in the distal ulnar epiphysis, the roentgenographic diagnosis could not be made.