A translocation of the proximal radius and ulna combined with a posterior dislocation of the elbow is quite rare. To the best of our knowledge, the only case with this condition, who had been treated using a closed method was reported by MacSween in 1978. This paper reports a ten-year-old girl who fell from a desk onto her outstretched left hand. The initial radiographs showed a simple posterior dislocation of the elbow. However, the radiographs taken after the closed reduction revealed a translocation of both forearm bones. It was possible to relocate both bones using a closed method, and the patient recovered from the associated ulnar nerve palsy at five weeks post-trauma. At the follow-up examination three months post-trauma, the nerve was found to be fully regenerated using electromyography and a nerve conduction study, and the patient regained the full range of elbow motion without pain. When a posterior dislocation of the elbow occurs, close attention is needed in order to detect the combined translocation of the proximal radio-ulnar joint. If treated early, a closed reduction leads to a good result. However, a careful physical examination and a thorough investigation of the radiographs are necessary.