PURPOSE: To assess the value of MR imaging in the diagnosis of medial collateral ligament injury of the knee, we used MR imaging to evaluate the characteristic findings in MCL tears and the frequency of associated knee joint injury. MATERIALS AND METHODS: We retrospectively reviewed 26 patients within four weeks of MCL injury, analysed MR findings and correlated them with surgical findings. We evaluated discontinuity, heterogeneous signal intensityof MCL, thin band- like low signal intensity at MCL, facial edema, loss of clear demarcation of adjacent fat andalso combined bone injury, meniscus injury and other ligament injury. RESULTS: Complete MCL tears were present in 14 patients and partial tears in 12. Complete tears showed discontinuity of MCL, fascial edema and loss of clear demarcation from adjacent fat in 11 patients (79%) ; proximal MCL tears are more common than distal tears. Partial tears showed thin band-like low signal intensity within MCL, fascial edema and loss of clear demarcation fromadjacent fat in seven patients (58%) ; all patients with MCL injury showed fascial edema ; in 12 patients there was loss of clear demarcation from adjacent fat. We could not, however, distinguish between complete tears and partial tears when MCL showed heterogeneous high signal intensity. Combined bone injury in MCL tears was found in eight patients (62%) ; the most common sites of this were the lateral femoral condyle and lateral tibial plateau. There was associated injury involving other ligaments(ACL:50%; PCL: 27%). Combined meniscus injury in MCL tearswas present in 17 patients and the most common meniscus site (50%) is the posterior horn of the medial meniscus. CONCLUSION: Complete MCL tears showed discontinuity of MCL and partial tears showed a thin band-like low signal intensity within MCL. All patients with MCL injury showed fascial edema, and loss of clear demarcation fromadjacent fat. Various other injuries combine with MCL tears. MR imaging is therefore useful in the evaluation ofmedial collateral ligament injury and associated knee joint injury.