PURPOSE: To assess the significance of alteration of the anterior cruciate ligament (ACL) axis as seen on magnetic resonance imaging of anterior cruciate ligament tear. MATERIALS AND METHODS: ACL angle (angle between lateral tibial plateau and ACL) and ACL-Blumensaat line angle (angle between ACL and Blumensaat line) were measured in 76 patients with ACL tear and 55 patients with normal ACL. The significance of the change in orientation of the ACL was evaluated. RESULTS: ACL angle was significantly smaller in the tear group (35.07 ° ± 10.34 °, mean±standard deviation) than in the normal group(54.11°±4.61°)(p<0.001); when cutoff value was less than 45 , sensitivity, specificity and accuracy were 85.5%, 98.2% and 90.8%, respectively. The ACL-Blumensaat line angle was significantly greater in the tear group (20.33°±9.56°) than in the normal group (2.55°±4.86°)(p<0.001). when cutoff value was more than 10°, sensitivity, specificity and accuracy were 90.8%, 94.5%, and 92.4%, repectively. CONCLUSION: The change in orientation of the ACL was an important associated finding in cases of ACL tear. We suggest that cutoff values should be an ACL angle of less than 45 , and an ACL-Blumensaat line angle of more than 10°.