The plain radiographic findings indicative of discoid lateral meniscus have been described as flattening of lateral femoral condyle, widened lateral joint space, cupping, of lateral aspect of the tibial plateau, high fibular head, oblique alignment of the articular surface of the lateral tibial condyle and minimal hypoplasia of lateral tibial spine. We analyzed the radiologic findings of the 21 knees of nineteen patients who had been confirmed as a lateral discoid meniscus with arthroscopic study from August, 1988 to April, 1991. The results were as follows:1. The 21 discoid lateral menisci (19 patients) were diagnosed by arthroscopic finding. Of these, 12 menisci (10 patients) were classified as incomplete and 9 menisci (9 patients) were classified as complete. 2. The age were ranged from 16 to 52. There were 13 men (62% ), 6 women (38%), 12 right, 5 left and 4 bilateral lesions. The four bilateral lesions were incomplete lateral discoid menisci. 3. The average distance between fibular head and lateral edge of tibial articular surface were 13. Omm in normal meniscus and 11.5mm in lateral discoid meniscus. The fibular head of lateral discoid meniscus was 1.5mm higher than normal meniscus. The fibular heads of 15 knees out of 21 discoid menisci were higher than normal average. 4. In plain anteroposterior rhentgenogram, the 7 knees out of 21 discoid menisci were identified as a flattening, but there wasn't any case of flattening of lateral femoral condyle in normal meniscus. 5. In normal meniscus, the average height of medial and lateral tibial spine were 9.4, 8.0mm and the medial tibial spine was 1.4mm longer than the lateral tibial spine. In discoid menis-cus, the average height of medial and lateral tibia spine were 8.8, 7.4mm and medial tibial spine was l. 4mm longer than lateral tibial spine. As a result, the average height of medial and lateral tibial spine of discoid meniscus was 0.6mm shorter than that of normal meniscus. 6. In normal meniscus, the average width of the medial and lateral joint space were 4.5, 5.0mm and lateral joint space was 0.5mm wider than the medial joint space. In lateral discoid meniscus, the average width of medial and lateral jont space was 4.5, 6.3mm, which happens to be the lateral joint space was 1.8mm wider than the medial joint space. 7. The 3 cases out of 22 normal menisci and 15 cases of 21 discoid menisci were identified as a cupping of lateral tibial plateau. 8. The 3 cases out of 21 discoid menisci had oblique alignment of articular surface of lateral tibial condyle, which was not found in normal meniscus. 9. In normal meniscus, there wasn't any case of laterl femoral condylar indentation that the depth of lateral femoral condylar indentation was deeper than 1.0mm. In lateral discoid meniscus, there were 7 cases (33%) out of 21 knees. So, femoral condylar indentation happens to be another finding of lateral discoid meniscus.