1. Two hundred and eleven cases of arthroplasty of the hip have been studied in an attempt to establish the causes of success and failure by comparing the excellent, good and bad results. 2. The findings suggest that the result of an arthroplasty depends largely upon four factors: the surgical approach to the hip joint, the acetabular roof, the interposition substance, and the early post-operative complications. 3. Some important points in the technique of operation and in the post-operative management are described.