We repaired the anterior cruciate ligament of the knee using a Dacron prosthesis in fifty-four patients. Nineteen patients had an acute injury of the anterior cruciate ligament, and thirty-five had a chronic rupture. In each patient, only one knee was involved. In twenty-six patients, the prosthesis was implanted arthroscopically, and in twenty-eight, the operation was done by arthrotomy. The length of follow-up ranged from two to five years. In thirty-one patients, a second arthroscopy of the knee was done at follow-up, and the findings were recorded. Only twenty-three patients were free of symptoms at the latest follow-up. Stability and the scores on the grading system of Lysholm and Gillquist were excellent in the first postoperative year; the results worsened due to complications occurring in the second year of follow-up. There were no differences in the results between patients who had a closed procedure and those who had an open procedure. No differences were noted between the patients who had an acute injury and those who had a chronic rupture. Twenty-six failures were directly related to the prosthesis, including rupture of the femoral insertion (thirteen) or tibial insertion (three), rupture of the central body (seven), and elongation in continuity (three). We concluded that a free Dacron prosthesis that is used to replace or supplement a torn anterior cruciate ligament is not a durable substitute for that ligament.