Twenty patients each with a fractured femoral neck had a prosthesis with a built-in ball-and-socket joint inserted. Movements at the built-in joint were compared with total hip movement immediately after operation, one month later and three months later. It was found that the built-in joint soon lost mobility and at three months was almost completely stiff. Consequently such joints cannot be expected to prevent the acetabular erosion which is liable to follow femoral head replacement for fracture.