We evaluated the thigh muscle strength using an isokinetic dynamometer in 80 patients 5 to 10 years following anterior cruciate ligament reconstruction. Patients with good results also had good muscle function, both of their flexor and their extensor muscles. Patients with poor results demonstrated reduced muscle strength in both their hamstrings and their quadriceps. Patients with fair functional results demonstrated reduced strength of their extensor muscles compared to their normal leg. However, strength in their operated leg's flexor muscles was unchanged compared to their normal leg. This may be due to the fact that strong hamstring muscles can compensate for some instability of the anterior cruciate ligament.