Thirteen posterior Monteggia fracture-dislocations in adults were treated surgically at the Massachusetts General Hospital from 1980 to 1988. A characteristic lesion was observed, consisting of a proximal ulna fracture with a triangular or quadrangular fracture at or near the level of the coronoid, a posterior or posterolateral radiocapitellar dislocation, and, in 10 cases, a radial head fracture. Nine patients were women and four were men, with an average age of 56 years. Following reduction of the radiocapitellar dislocation, the ulnar fractures were treated with plates in each case. Seven fractured radial heads were excised, one replaced with a silicone prosthesis, and three treated by open reduction and internal fixation. The 11 surviving patients were observed using the performance index of Broberg and Morrey at an average follow-up time of 38.4 months. The conditions of three were rated excellent, three good, four fair, and one poor. Incomplete reduction of the ulnar fracture with residual posterior radiocapitellar subluxation was observed in four cases, all leading to loss of forearm supination. We believe this lesion to be more common than previously reported. Recognition of its specific anatomic features is essential to achieve a functional outcome.