During the years 1971 through mid-1977, approximately 3,000 total hip replacements were performed at The Hospital for Special Surgery using metal-to-plastic prostheses. During that period of time, thirty-five operations were done for mechanical failure in the absence of infection. The mechanical failures were distributed as follows: Fourteen patients had dislocations of the prostheses and seven patients had loosening of the femoral component, mostly with varus positioning of the femoral stem. Six patients, most of them heavy, active men, had fractures of the femoral stem. Three patients had loosening of the acetabular component. (In two of three there was insufficient bone stock over the acetabulum.) Four patients had proximal migration of the greater trochanter and one had a fracture-dislocation of the acetabulum. In all cases a technical or anatomical problem could be identified as the reason for mechanical failure necessitating reoperation. By careful preoperative planning, proper selection of procedures and positioning of prostheses, bone-grafting when necessary, and stricter attention to the technique of using cement, we could conceivably have avoided two-thirds of these failures.