The KT-1000 arthrometer was used to measure anterior tibial displacement in patients with acute unilateral anterior cruciate ligament (ACL) disruptions. Measurement revealed 11.5 mm absolute displacement on the injured side and 3.6 mm side-to-side difference with the 89 N test, 15.2 mm absolute displacement and 6.1 mm side-to-side difference with the manual maximum displacement test, and 8.5 mm displacement and 2.0 mm side-to-side difference with the active quadriceps test. Chronic anterior instabilities measured 13.0 mm absolute displacement on the involved side and 5.4 mm side-to-side difference with the 89N test, 17.7 mm displacement and 8.6 mm side-to-side difference with the manual maximum displacement test, 10.8 mm displacement and 4.4 mm side-to-side difference with the active quadriceps test. Testing technique, measurements in control subjects without previous knee injuries, and displacement measurements of other authors and other instrumented testing devices are described.