The purpose of this prospective study was to quantify and compare the amount of anterior tibial translation (ATT) occurring in ACL-reconstructed knees during both a static passive Lachman test and an isokinetic knee extension exercise, pre- and postoperatively. Stress-radiography combined with an electrogoniometer system was applied to 49 knees before and after ACL reconstruction. The Lysholm score was calculated and subjective evaluation assessed before operation and at follow-up. Both measurement methods confirmed a significant decrease of ATT after surgery. Side-to-side differences in ATT were seen in the passive Lachman test postoperatively, and were not found during isokinetic extension from 90 degrees to 0 degree. There was no significant correlation between static passive stability and the functional knee score at follow-up. In addition, the patients with a more than 3 mm side-to-side difference in the passive Lachman test after surgery, showed less than a 1 mm side-to-side difference during isokinetic exercise at a flexion angle of 20 degrees. These results suggest that ACL reconstruction improves ATT in both tests, but the side-to-side difference is greater with the static Lachman test.