1. A concept and design of polycentric knee arthroplasty based on the biomechanics of normal knee movement is presented. The diseased articular surfaces of the femoral condyles and tibial plateaus are replaced separately by prosthetic implants secured with cement. The collateral and cruciate ligaments are retained to maintain joint stability. 2. The early results from this arthroplasty in twenty-two knees are assessed. The operation gave relief of pain in twenty-two, a range of movement greater than 90 degrees in thirteen, no lateral instability in sixteen of twenty-two knees and an increased degree of mobility for nineteen of twenty individuals (two bilateral arthroplasties). 3. Operative complications consisted of delayed wound healing in four knees and common peroneal nerve palsy in one. One knee was subsequently arthrodesed for lack of functional improvement.