Sixteen patients with traumatic injury to the knees were surveyed with magnetic resonance (MR) machines with field strength of either 0.5 Tesla (T) using head coil (10 cases) or 1.5 T using dedicated knee coil (6 cases). Tears of the anterior cruciate ligament (ACL) were found in eight cases. The other eight cases had injuries of the other ligaments, meniscus or bone with intact ACL. All cases were proved by arthroscopy. Primary and secondary signs of the ACL tears were evaluated by MR imaging. Primary signs included the status of ACL. Secondary signs consisted of bone bruise in the posterolateral tibial plateau, in the lateral femoral condyle, or both (so-called kissing bone bruise), fracture at the posterolateral tibial plateau, Segond fracture, anterior dislocation of the tibia (> 5 mm), buckled posterior cruciate ligament (PCL), positive PCL line and the deep lateral femoral notch (> 1.5 mm). Of the eight cases of the ACL tears, 5 cases (62.5%) revealed thickening and edema of the ACL, 3 cases (37.5%) kissing bone bruise, 1 case (12.5%) Segond fracture, 2 cases (25%) buckled PCL, 2 cases (25%) deep lateral femoral notch, 2 cases (25%) anterior dislocation of the tibia and 0 case (0%) positive PCL line. The ACL had normal thickness of low signal intensity in 3 cases of chronic ACL tears (50%). Lateral meniscal tears were found in seven of the eight cases of ACL tears (87.5%). For the eight cases with intact ACL, the ACL was normal in thickness and of low signal intensity. Other findings were bone bruise in the lateral femoral condyle (1 case, 12.5%), fracture at the posterolateral tibial plateau (1 case, 12.5%), buckled PCL (1 case, 12.5%) and positive PCL line (1 case, 12.5%). It is concluded that primary sign is important in the MR diagnosis of the ACL tears. Secondary signs are helpful but may be present in lesions with intact ACL.