One hundred and twenty-seven hips in 125 adults were treated for traumatic posterior dislocation during a period of two decades. Ninety-eight patients (100 hips) were available for follow-up examination at a minimum of five years (average, fourteen years) after injury. By both clinical and radiographic criteria eighty hips (80 per cent) had an excellent or good result. Forty-two per cent of the hips that were reduced more than six hours after the dislocation had an excellent or good result and 88 per cent of those that were reduced within six hours after dislocation had an excellent or good result. Thirty-one per cent of the hips with a Grade-III dislocation had an excellent or good result, as compared with 90 per cent of those with a Grade-I dislocation. Twenty-two per cent of the hips with an excellent or good result had avascular necrosis of the femoral head. We found the time-interval between injury and reduction, the severity of the initial injury, and the development of avascular necrosis of the femoral head to be the most important factors with regard to the long-term prognosis. Avascular necrosis occurred most frequently in hips that underwent reduction after a delay of more than six hours.