This is an analysis of 52 fractures of the posterior wall of the acetabulum treated operatively and reviewed 2-15 years after injury. In 48 cases the fracture was associated with posterior dislocation of the hip, which was treated by closed reduction soon after the injury. In all but two of the cases there was a displaced single or comminuted fragment resulting in a large defect in the posterior acetabular wall; they were treated by open reduction and internal fixation to restore joint congruity and stability. In the remaining two cases, the fragment was small but trapped in the joint and was excised. A strict correlation was found between accurate reduction of the fracture and the clinical and radiological results, which were excellent or very good in 85 per cent and 87.5 per cent of the patients, respectively. Surgical and late complications were peroneal palsy in four patients, ectopic ossification restricting hip movement in two cases, aseptic necrosis in three, and osteoarthritis in another three cases.