The KT-1000 and similar non-invasive arthrometers are used as a complement to clinical examination in the diagnosis of anterior cruciate ligament (ACL) rupture and during the follow-up after surgery. We compared the two methods, KT-1000 and Radiostereometric analysis (RSA), when used to measure anterior-posterior knee laxity (A-P laxity) in patients with ACL rupture, before and after the reconstruction of this ligament, in a prospective, comparative study. Twenty-two consecutive patients (14 men, 8 women) with a median age of 24 years (range 16-41) were studied. All the patients had a unilateral ACL rupture and an intact contralateral knee. The patients were operated on by one experienced surgeon using the bone-patellar tendon-bone (BTB) autograft. Preoperatively and 2 years after the reconstruction, all the patients were evaluated using KT-1000 and RSA measurements of A-P laxity. The side-to-side differences between the injured and the intact knees, that is, total A-P laxity for both knees, are presented. Preoperatively, the median side-to-side differences using the two methods (KT-1000/RSA) were 4.0 (0-10)/7.4 mm (2.2-17.4) (P<0.0001). The total A-P laxity on the injured side was 11.0 (6.0-18.0)/10.9 mm (6.2-19.6) (n.s), while it was 8.0 (6.0-10.0)/3.1 mm (0.2-8.6) on the intact side (P<0.0001). A side-to-side difference of more than 3.0 mm was defined as the cut-off value for indicating ACL rupture. Using the KT-1000, 11 of 22 (50%) patients had a cut-off value above 3.0 mm, while the corresponding figure for RSA was 21/22 (95%) patients. At the 2-year follow-up, the median side-to-side differences using the two methods (KT-1000/RSA) were 0.5 (-1.5 to 4.0)/2.8 mm (-1.8 to 10.7) (P<0.0001). The total A-P laxity on the operated side was 9.5 (7.5-14.0)/6.5 mm (2.4-14.1) (P<0.0001). We conclude that the KT-1000 recorded significantly smaller side-to-side differences than did the RSA, both before and after the reconstruction of the ACL using a BTB autograft. Before it was mainly an effect of larger A-P laxity recordings with KT-1000 on the intact side, and after the reconstruction, the KT-1000 still recorded larger A-P laxity on the intact side and also larger A-P laxity on the reconstructed side than RSA.