Arthroscopic surgery of the knee is now a widely performed orthopedic technique. Of this, arthrosoopic surgery of discoid meniscus is one of the most technically demanding procedures of this field. In our experience, discoid meniscus was appeared to be frequently accompanied with villous hypertrophied synovium, which obstructed the arthroscopic view. The purpose of this study is to prove the presence of synovial hypertrophy in discoid meniscus and to develop an adequate portal of entry for successful arthroscopic surgery of discoid meniscus. 155 cases of arthroscopy of the knee were performed at the Department of Orthopedic Surgery, College of Medicine, Seoul National University between January 1987 and March 1988. Of these, 25 cases were discoid menisci and they were all lateral. Operative findings were observed through arthroscopy and they were also reviewed by recorded video tapes in 25 cases of arthroscopic surgery of discoid meniscus and 57 cases of non-discoid meniscal lesions for the presence of synovial hypertrophy. Synovial biopsy was performed in selected cases with synovial hypertrophy for observation of histological findings. Hypertrophied synovium in discoid meniscus was mainly localized in anterior compartment and intercondylar area of tibia. Therefore, when conventional anterolateral or anteromedial portals were used, much difficulty was confronted because hypertrophied synovium obstructed the visual field and crowding with surgical instruments was occurred in restricted space of discoid meniscus. Much more clear visual field was obtained when the medial midpatellar portal was used instead of conventional portals. The conclusions of this study are as follows : 1. The incidence of discoid meniscus was 16.1%. 2. Synovial hypertrophy was present in all 25 cases of discoid meniscus and 21 cases among 57 non-discoid meniscal lesions(35%) on operative findings 3. Synovial biopsy of hypertrophied synovium showed irritative nonspecific inflammatory lesions but there was no histologicsl differences between discoid and non-discoid meniscus. 4. Medial midpatellar portal is considered to be better approach than conventional portals for clear visual field and less crowding with instruments in arthroscopic surgery of lateral discoid meniscus.