This study concerns the frequency of para-articular ossifications after primary prosthetic replacement ad modum Austin T. Moore in the treatment of unstable fractures of the femoral neck. In 121 patients who were X-rayed after surgery ossifications were found in 45 per cent of the cases. Patients that preoperatively had severe coxarthritis or rheumatoid arthritis are excluded. Two thirds of the bone formations described were very small and without clinical importance. About 20 per cent of the ossifications were of a considerable size, but no clinical consequences could be demonstrated. About 12 per cent of the ossifications--6 per cent of all the patients in this study--were large, and only in this group could a compromised hip function be found. It seems as the process of ossification is limited to the first 3 postoperative months. There is a good correlation between the communications regarding ectopic bone formation after total hip replacement and the frequency found in this study.