There are three types of plica known in knee joint, namely suprapatellar plica, mediopatellar plica and infrapatellar plica. The most important of these three from clinical point of view is mediopatellar plica, since that it has been noticed definitely as one of major causes of internal derangement in increasing frequency recently with help from arthroscopy. The authors could observe 39 cases of mediopatellar plica syndrome per se out of 155 arthroscopies of knee joint from September, 1983 to January, 1986. The results of clinical analysis upon these cases are as follows. 1. The presence rate of mediopatellar plica against total arthroscopies was 61% (95/155 cases) regardless of its clinical significance. 2. The number of mediopatellar plica syndrome excluding other causes of internal derangement was 39 cases which is 25.2% of total arthroscopies. The accuracy rate of prearthroscopic diagnosis for mediopatellar plica syndrome was 36%(14/39 cases). 4. The chondromalacia change if present in association with mediopatellar plica was found mostly in medial condylar surface of femur and its corresponding surface of patellar. 5. The treatment modalities applied for these proven mediopatellar plica syndrome were conservative for 4 cases, arthroscopic excision for 27 cases and excision through limited arthrotomy for 8 cases. The results of treatment at more than 1 year of follow-up were generally good having 23 cases in “good” grade, 14 cases in “fair” grade and only 2 cases in “poor” grade according to Munzinger's assessment scheme. 6. Arthroscopy was found most valuable method for confirmatory diagnosis and treatment of mediopatellar syndrome.