Arthroscopy of the knee joint is an essential diagnostic procedure of the knee joint lesions because of its high diagnostic accuracy and simple procedure and high safety. We analysed in 300 cases of knee joints arthroscopy which performed in Kyung Hee University Hospital from July 1978 to June 1982. The results were as follows: 1. The diagnostic accuracy of clinical diagnosis of knee joint lesions based on arthroscopic findings was 51.0%. 2. The diagnostic accuracy of clinical diagnosis in surgically confirmed cases of knee joint lesions was 56.1% and diagnostic accuracy of arthroscopy was 90.9%. 3. In meniscal lesions, the diagnostic accuracy of arthroscopic diagnosis was 94.5%, arthrographic diagnostic accuracy was 84.9%, clinical diagnostic accuracy was 48.9% and combined accuracy of three diagnostic methods was 97.3%. 4. In cases which diagnosed as I.D.K. (Internal Derangement of Knee) in initial stage of knee joint lesions, meniscal lesions (32.8%) and ligament injuries (22.4%) were predominant lessions based on arthroscopic findings. 5. The frequency of meniscal tear were no significant difference in lateral (33.3%) or medial (36.0%) meniscus except discoid meniscus rupture. 6. 68 Cases of 300 arthroscopies (22.7%) were avoided unncessary arthrotomy through preoperative arthroscopic examination of knee joint. 7. Arthroscopy is very useful and essential not only to confirm the diagnosis but also to treatment of knee joint lesions.