A non-constrained total knee prosthesis designed at St. Luc Hospital, Montreal, has been used there since 1977. The geometry of its femoral component simulates the normal anatomy of the knee and the flat articular surfaces of its tibial component allow unconstrained rotation, rolling, and sliding motion, as dictated by the retained cruciate ligaments. The results achieved in the first 107 consecutive arthroplasties (in eighty-three patients) were evaluated after a minimum follow-up of two years (range, two to four and one-half years). Using the knee-rating system of The Hospital for Special Surgery, 91 per cent of the knees had an excellent or good result. In ninety-eight knees the preoperative pain was adequately relieved and the limb was in physiological valgus alignment. One hundred and four knees had normal medial-lateral stability in extension, and three were unstable. The average postoperative arc of flexion was 102.8 degrees. Sixty-three patients (76 per cent) could ascend and descend stairs normally, and gait studies on seventeen of these patients (twenty-three involved knees) demonstrated that the flexion arc of the knee while the patients were ascending and descending stairs was the same as that of normal controls. Three knees had a deep wound infection and one knee had aseptic loosening of the femoral component.