Great judgment should be exercised in the selection of patients for replacement of the femoral head and a careful postoperative program should be planned. The importance of partial weight-bearing for a period of at least six months should be emphasized, with a gradual increase as roentgenograms show the reaction of bone about the prosthesis. New-bone formation takes place within the fenestrations of the prosthesis. Once this has occurred, the stem cannot be removed without chiseling away the new bone. The prosthesis becomes, literally, self-locking. It is very interesting to see, roentgenographically, the increasing density of bone in and under the prosthesis in the stress-bearing locations. If a prosthesis is to last indefinitely, it must be constructed in such a way that stress is distributed so as to produce a dynamic reaction, in accordance with Wolff's law. In this way, there gradually occurs a thickening of bone about the prosthesis enabling it to carry the normal load of weight and stress. The prosthesis becomes a part of the living bone. Five years ago a preliminary report was made of our experience with a self-locking Vitallium prosthesis in thirty-three patiensts2. The present report reviews the same experience in 153 patients and in 159 operations during a period of six years. There have been no fractures of the bone in applying the prosthesis, and no dislocations of the hip have occurred during the operation. Apparently Vitallium is insert in the tissues, and it is possible that the prosthesis may last indefinitely. Patients are made to understand that no artificial replacement can be expected to function as well as the normal hip and are urged to be careful. They are advised to always carry a cane or to use a light support. An attempt is being made to follow all patients. Time has already shown conclusively that transplanted cancellous bone in the medullary canal undergoes metaplastic changes and that the bone becomes progressively denser in the areas of stress. Time has also shown, as was anticipated, that there is hyperplasia of bone in, under, and about the prosthesis when weight-bearing is properly applied. Excessive weight-bearing stress or too short a stem results in pressure necrosis with a loose, clicking prosthesis and pain. With the stimulating stress of gradual weight-bearing, areas of increased density of bone appear about the prosthesis consistent with known mechanical and engineering principles. When patients have been properly selected and when operation has been correctly performed, and postoperative follow-up care has been conscientiously carried out, this prosthesis appears to become a part of the femur and to carry the normal stress load of weight-bearing with minimal degenerative changes and with minimal pain. The results so far are encouraging, and the increasing number of patients bears witness to growing confidence in the procedure. Final results can be known and a thorough evaluation of the method can be made only after many years of experience.