Magnetic resonance imaging(MRI) is accepted as a useful tool for the diagnosis of injury of the knee recently. The anterior cruciate ligament tear is accompanied by the damage of the osseous and soft tissue of the knee. The occult osseous lesion is found on magnetic resonance imaging as bone density change that is not found on the simple radiograph. From Jan. 1992 to Apr. 1994, magnetic resonance imaging was performed on 44 patients with acute anterior cruciate ligament tears. MRI was done within 6 weeks of the index anterior cruciate ligament injury. The criteria of decreased signal intensity on the Tl weighted image and increased signal intensity on the T2 weighted image was applied for the incidence assessment of the occult osseous lesion. After arthroscopy with or without ACL reconstruction, follow-up simple radiograph was checked every 3 months. Occult osseous lesions were documented in 32(73%) of the 44 patients. Of the 32 patients with bony lesion, 81% had lesions of the lateral compartment. Lateral femoral condyle was involved in 15 cases(46.9%), and lateral tibial plateau in 22 cases (68.8%). Follow-up X-ray revealed no joint deperession in all of the 32 patients with the occult osseous lesion. This result suggests that there is a high correlation between the occult osseous lesion of lateral compartment of the knee on magnetic resonance imaging and anterior cruciate ligament tear. The presence of the occult osseous lesion in the patient with acute anterior cruciate ligament tear did not affect rehabilitation and weight bearing.