The authors have observed that the systematic use of arthroscopy results in the discovery of a number of isolated ruptures of the anterior cruciate ligament which could not be diagnosed by clinical examination alone. Twelve such cases have been operated on. The authors believe that the initial trauma was not one of hyper-extension or of forced internal rotation of the tibia. They have shown by biomechanical studies that it was due to an excessive contraction of the quadriceps on the weight-bearing limb with the tibia in internal rotation. They recommend the early use of tendons such as the patellar tendon for the repair of the ligament.