Twenty-five patients had failure of a metal-backed patellar component after total knee replacement. Five manufacturers and seven designs were involved. There was no apparent correlation between failure of the component and the age or sex of the patient, the diagnosis, the use of cement, the femorotibial alignment, or the use of lateral release. The patients in whom the patellar implant failed were relatively heavy, and the diagnosis in most of them was osteoarthritis. The failure was due to one of two mechanisms: wear or fracture, or both, of the polyethylene over the edge of the metal backing (eighteen components), or dissociation of the polyethylene or the base-plate, or both, from the anchoring pegs (seven components). In many of the patients, failure of the component was not suspected before arthrotomy. The failure led to considerable wear of the femoral component in eleven patients and to metal-induced synovitis in twenty-three. We concluded that metal backing may predispose the patellar component to a small but important likelihood of failure, and we urge caution in choosing a metal-backed patellar implant. Additional research is necessary to improve designs for the patellar component, especially if metal backing is to be used.