Twenty patients who had substitution of the anterior cruciate ligament with a Gore-Tex synthetic ligament were evaluated preoperatively and postoperatively with the University of California at Los Angeles instrumented clinical-testing apparatus, which records anterior-posterior force versus displacement-response curves of the tibia with respect to the femur at 20 degrees of flexion of the knee. The mean age of the patients was thirty-three years (range, nineteen to fifty-four years). The duration of follow-up ranged from twenty-four to forty-four months (mean, thirty-one months). The mean preoperative difference in anterior laxity between the injured knee and the normal knee (4.5 millimeters with neutral rotation of the foot) was unchanged two years after the operation; at that time, all patients had an anterior laxity of the injured knee of more than eight millimeters, and 90 per cent had a difference in anterior laxity of more than two millimeters between sides. The mean values for anterior stiffness at fifty and 100 newtons of anterior force were unchanged after the operation, remaining at 40 to 50 per cent of normal levels. At 200 newtons, or 20.4 kilograms (forty-five pounds) of anterior force, the mean stiffness of the involved knee was 11 to 17 per cent greater than that of the normal knee. Clinically, there were improvements in both subjective and objective knee-rating scores. All but four patients had a reduction of at least one grade in the pivot-shift score; in thirteen, the pivot-shift sign was eliminated.(ABSTRACT TRUNCATED AT 250 WORDS)