Numerous factors conspire to make hip replacement arthroplasty for rheumatoid arthritis different from the same operation in osteo-arthritis. Patients are frequently younger, arousing fears that the patient will outlive the prosthesis. However this factor is more than counterbalanced by the constraints imposed by the disease in other joints: polyarticular involvement usually protects the hip arthroplasty from weight-bearing stress thus prolonging its life. Protrusio acetabuli, uncommon in osteo-arthritis is the commonest presentation in rheumatoid arthritis because of the concentric cartilage degradation which erodes the acetabulum medially and proximally. In arthroplasty for protrusio, a plea is made for grafting of the floor of the acetabulum with solid bone derived usually from the femoral head. This offers a stronger and more physiological support for the acetabular component than synthetic devices. The hip is often only one of the many joints requiring arthroplasty to maintain locomotor function in severe cases of rheumatoid and the results of multiple operations are usually very gratifying. However, any form of surgery may be contra-indicated in a patient enfeebled by prolonged immobilisation from involvement of multiple joints. Late infection of cemented prosthesis is commoner in rheumatoid than in osteo-arthritis: such patients need antibiotic cover during transient bacteraemia from any cause such as dental extraction, trivial infections etc.