We studied the clinical values of the MRI examinations of 19 knees in 19 patients with arthroscopically documented discoid lateral menisci of the knee joints from Jun. 1991 to Nov. 1992 at the department of orthopaedic surgery of the school of medicine in Kyung Hee University. The purpose of this study is to demonstrate the diagnostic value of MRI for discoid lateral meniscus and the finding of MRI according the type of tear and cause of misdiagnosis. Average age of patients was 22.7 years (range from 7 years to 52 years). The 12 knees had trauma history and another 7 knees without truma history. Types of tear were peripheral tears (7 cases), multiple tears (6 cases with 5 peripheral detachment and horizontal tears). We defined MRI criteria for the diagnosis of discoid menisci by the three or more 5mm thickness in the sagittal image or more 15mm width at mid-horn in the coronal 1mage11.12) 13 out of 19 discoid lateral menisci (68.4%) were correctly diagnosed preopratively by MRI, however 6 were diagnosed as lateral meniscal tear. It was very difficult to diagnose discoid lateral meniscus which had peripheral detachment by MRI. 5 out of 6 misdiagnosed discoid lateral menisci had peripheral dettachment from anterior and midhorn and displaced posterocentrally. They were diagnosed as lateral meniscal injury by the finding of MRI. Another one misdiagnosis was double transverse tear. The differentiation between the complete and incomplete discoid meniscus with the MR imaging was performed. Complete discoid meniscus was defined as the square shape image in the sagittal and coronal image of MRI was extended into the entire surface of tibial condyle. Incomplete discoid meniscus was defined as the size of discoid meniscus was larger than the normal lateral meniscus, but the medial portion of discoid meniscus in the coronal image was triangular shape. We concluded that it was important to decide treatment mordality preoperatively after the evaluation of MRI about the type and severity of injury.