The purpose of the study was to examine anterior tibial translation (ATT) and hamstring activity patterns during active and passive arthrometric knee laxity assessment. Knee laxity of 12 chronic ACL-deficient and 12 normal subjects was assessed using a Dynamic Cruciate Tester arthrometer during five active maximal isometric quadriceps contractions (active tests) and five passive tests (240 N applied load). Electromyographic activity of semimembranosus and biceps femoris were sampled during testing. The right-left knee laxity difference of control subjects did not alter between test types (active x = 0.6 mm; passive x = 0.4 mm). However, the average involved/non-involved knee laxity difference of ACL-deficient subjects was greater in passive (x = 4.0 mm) compared with active tests (x = 0.9 mm). Hamstring activity varied widely within both subject groups during active testing. It was concluded that hamstring activity should be monitored during arthrometric assessment of knee laxity to ensure reliable passive ATT results and to provide insight into knee stabilization strategies used by ACL-deficient patients during active tests. © 1995.