Abstract A consecutive series of 106 patients with displaced femoral neck fractures was treated initially with Moore prostheses. The patients' mean age was 82.5 years (range, 67–91). At follow-up after two years, 71 patients attended. Thirty-seven per cent of these were in need of a conversion of their one-piece device to a total hip replacement on account of hip pain. When only active patients living in their own homes were considered, 55 per cent were found to need total hip replacements. The main reason for the development of hip pain was acetabular derangement. It was concluded that active patients, disregarding their age, should not be treated with one piece prostheses if their fractures could be satisfactorily reduced and pinned. If this cannot be accomplished, a primary total hip replacement may be considered.