The KT-1000 is widely accepted as a tool for the instrumented measurement of the antero-posterior (AP) tibial translation. The aim of this study is to compare the data obtained with the KT-1000 in ACL deficient knees with the data obtained using a navigation system during "in vivo" ACL reconstruction procedures and to validate the accuracy of the KT-1000. An ACL reconstruction was performed using computer aided surgical navigation (Orthopilot, B-Braun, Aesculap, Tuttlingen, Germany) in 30 patients. AP laxity measurements were obtained for all patients using KT-1000 arthrometer (in a conscious state and under general anaesthesia) and during surgery using the navigation system, always at 30 degrees of knee flexion. The mean AP translation was 14 +/- 4 and 15.6 +/- 3.8 mm using the KT-1000 in conscious and under general anaesthesia, respectively (P = 0.02) and 16.1 +/- 3.7 mm using navigation. Measurements obtained with the KT-1000 under general anaesthesia were no different from those obtained "in vivo" with the navigation system (P = 0.37). In conclusion this study validates the accuracy of the KT-1000 to exactly calculate AP translation of the tibia, in comparison with the more accurate measurements obtained using a navigation system.