In a biomechanical and morphological study of cadaver knees and a clinical end-result study of fifty patients with tears of the anterior cruciate ligament, the structure and function of the ligament, the mechanism of injury, and the results of various forms of therapy were analyzed. It was concluded that tension of the anterior cruciate ligament varies during knee motion, being least at from 40 to 50 degrees of flexion; that there are adequate intraligamentous blood vessels and nerves for healing to occur; that isolated tears of the anterior cruciate ligament do occur (probably as the result of internal-rotation displacement of the tibia with respect to the femur); that such tears are frequently associated with meniscal tears; and that early results of operative or non-operative treatment of the anterior cruciate ligament are acceptable although late instability may occur.