The underlying study is a cross sectional study on the reliability of the KT1000 arthrometer and the Lachman test to determine the within-session inter-rater reliability and intra-rater reliability of the KT1000 arthrometer and the Lachman test. Twenty patients with a complete tear of the anterior cruciate ligament (ACL) were examined in a single session each. During the assessment, two physical therapists measured the anterior-posterior translation of the knee using both the KT1000 arthrometer and the Lachman test. One examiner performed a repeated measurement of each test for determination of intra-rater reliability. The examiners were blinded to the findings of their colleague. The intraclass correlation coefficient (ICC) was used to describe the degree of reliability of the measurements. High ICCs were found for the intra-rater reliability and the inter-rater reliability of the Lachman test (ICC=1.0 and 0.77). For the KT1000 arthrometer both ICCs were clearly lower (ICC=0.47 and 0.14). The KT1000 arthrometer shows inadequate reliabilities, even when measurements are repeated within a single measurement session. Contrastingly, the Lachman test is a reliable measurement to determine the anterior-posterior laxity of the ACL deficit knee. The results of the present study suggest good within-session intra-rater reliability as well as inter-rater reliability for the Lachman test.