Evaluation of muscle function can be helpful in rehabilitation programs with knee injuries. The purposes of this study were to evaluate muscle performance and functional level through eccentric and concentric isokinetic testing after anterior cruciate ligament reconstruction, and to determine whether eccentric and concentric isokinetic values were correlated with functional level using the Modified Cincinnati Scale (MCS). In order to compare the characteristics of eccentric and concentric parameters, we tested 24 male patients (mean age; 31.1 years) after primary reconstruction of ruptured anterior cruciate ligaments of the knee (mean duration; 20.2 months), at angular velocities of 60 degree/sec using Cybex 6000 isokinetic dynamometer. We also evaluated the functional level using the MCS. The values of peak torque of the involved knee extensors and flexors were significantly lower than the uninvolved limb in all eccentric and concentric tests (p < 0.01). The deficiency ratios of peak torque in knee extensors were significantly larger than knee flexors in both eccentric and concentric tests (p < 0.01). The MCS was inversely correlated with the deficiency ratio in peak torque of eccentric knee extensors (p < 0.05). These results suggest that eccentric knee extensor training is essential to restore the functional capacity of the injured knee and that isokinetic evaluation is necessary to plan rehabilitation programs to correct possible imbalances which may be predispose subjects to future injury.