We investigated that the anterior horn of lateral meniscus was well fitted on the lateral femoral condylar notch at full extension of knee through fresh cadaver study and arthroscopic study. We analyzed the radiological study of the 112 knees of patients who had been confirmed in diagnosis with arthroscopic study from August 1989 to April 1991. The results were as followings:1. Among 112 cases with arthroscopic study, 20 cases(17.9%) were lateral discoid meniscus, 23 cases were ACL lesion, and 69 cases were the other diagnosis. 2. Fifteen cases out of 20 cases lateral discoid meniscus were incomplete type, and 5 cases were complete type. 3. In the 20 cases of lateral discoid meniscus, 5 cases(25.0%) were deeper than 1.0mm for lateral notch sign. And all these 5 cases were incomplete type. As compared with lateral discoid meniscus, 3 cases(13.0%) in 23 cases of ACL lesion, 5 cases(7.2%) in 69 cases of the other diagnosis were deeper than 1.0mm. From the results we believed that the lateral notch sign was another significant sign for lateral discoid meniscus by radiologic method.