Cybex evaluation of muscle strength was performed on 15 patients with chronic anterior cruciate ligament tears following arthroscopic reconstruction using central one-third patella tendon autograft. Cybex testing was done preoperatively and at 3, 6, 9, and 12 months postoperatively. Peak torque and total work of the quadriceps and hamstrings at the angular velocity of 60 degree/sec and 180 degree/sec were measured and analyzed. The results were as follows: l. At the angular velocity of 60 degree/sec, the muscle strength of the quadriceps showed 45.4% deficit in peak torque and 50.9% deficit in total work compared to the contralateral normal knee preoperatively, 48.2% and 49.0% deficit respectively at 3 months after operation, 40.3% and 37.7% deficit at 6 months, 30.6% and 32.5% deficit at 9 months and 23.4% and 24.7% deficit at 1 year after operation. 2. At the angular velocity of 60 degree/sec, the muscle strenggth of the hamstrings showed 32.3% deficit in peak torque and 42.9% deficit in total work compared to the contralateral normal knee preoperatively, 39.3% and 42.4% deficit respectively at 3 months after operation, 27.3% and 32.3% deficit at 6 months, 21.2% and 22.3% deficit at 9 months and 17.9% and 18.4% deficit at 1 year after operation. 3. At the angular velocity of 180 degree/sec, the muscle strength of the quadriceps showed 38.4% deficit in peak torque and 42.0% deficit in total work compared to the contralateral normal knee preoperatively, 41.1% and 47.54% deficit respectively at 3 months after operation, 34.4% and 33.7% deficit at 6 months, 28.5% and 28.6% deficit at 9 months and 21.0% and 22.1% at 1 year after operation. 4. At the angular velocity of 180 degree/sec, the muscle strength of the hamstrings showed 26.9% deficit in peak torque and 31.8% deficit in total work compared to the contralateral normal knee preoperatively, 41.2% and 45.2% deficit respectively at 3 months after operation, 32.3% and 36.0% deficit at 6 months, 21.4% and 25.1% deficit at 9 months and 16.1% and 15.7% at 1 year after operation.