A prothesis has been designed to replace the articulating surfaces of the human ankle joint. The prothesis is in two parts, each forming a segment of a right circular cylinder with a single axis of rotation. The concave tibial component is manufactured from ultra-high molecular weight polyethylene and the talar component is manufactured from medical grade stainless steel. It is likely, however, that the talar component will be commercially manufactured from cobalt chrome alloy (Vitallium or Vinertia). The two components are secured to the cancellous bone by polymethylmethacrylate bone cement and laboratory tests have indicated that the bond should be strong enough to withstand the loads encountered at the ankle joint in vivo. The tests have also shown that the stability and strength of the ankle are not seriously reduced by implantation of the prosthesis. Laboratory wear tests and clinical experience over the last two years encourage optimism over the long term performance of the prothesis.