Specific bone signal abnormalities of MRI of patient with knee injury, although plain radiographs are normal, suggest that ACL injury may be present. Twenty eight MRI examinations in which ACL injury were present, were retrospectively reviewed and were correlated with bone signal changes. The results obtained were as follows; 1. Of twenty eight ACL injuries seen with MR imaging, bone signal abnormality was present at the lateral femoral condyle (LFC) in 8 patients (29%), at the posterolateral tibial plateau in 10patients (36%) and at both sites in 5 patients (18%). 2. In acute knee injury, all seven patients with complete ACL tear had bone signal abnormalities at MRI and of all seven patients, six patients showed bone signal abnormalities at lateral femoral condylar or posterolateral tibial plateau. 3. In conclusion, bone signal abnormalities at posterolateral tibia or lateral femoral condyle are present in almost cases of acute complete ACL tear. Therefore the bone signal abnormalities suggest that there may be ACL injury.