Objective To establish a quantitative biomechanical relationship between the anterior cruciate ligament deficiency and the posterior horn of medial meniscus. Methods 10 human cadaveric knees were examined using an in vitro knee-testing apparatus. In response to a combined 134 N anterior and 200 N axial compressive tibial load, the in situ forces in the posterior horn of medial meniscus were measured. Testing was performed on 10 knees at multiple angles of flexion (30?, 60?, 90?) before and after resection of anterior cruciate ligament. Results The resultant forces on the posterior horn of medial meniscus were as follows: intact ACL knees, (22.8?11.5) N in 30? flexion, (27.1?16.3) N in 60? flexion, (26.7?14.5) N in 90? flexion. ACL deficiency knees, (87.3?43.9) N in 30? flexion, (77.7?43.3) N in 60? flexion, (66.2?40.1) N in 90? flexion. The resultant forces significantly increased as a result of ACL deficiency(P