Four failed polycentric total knee prostheses were evaluated by clinical examination of the patients and surface analysis of the removed prosthetic components. The failures were secondary to a poor relationship of the prosthetic components in three cases and to abutment of the tibial spine against the femoral condyle in the fourth. A poor relationship of the prosthetic components was not necessarily secondary to malpositioning, because the positioning may have been accurate but the relationships became abnormal when the knee was unstable. Scanning electron-microscope examination revealed various wear patterns but no significant correlation was noted between the morphological type and the degree of wear. Accurate placement of the components and restoration of knee stability were critical requirements for the polycentric total knee prosthesis to be successful.