The synovial plica was first described in a cadaver study by Mayeda in 1918 and Iino first investigated arthroscopically in 1939. Thereafter the more attention had been put on the structure plica. But there was no universal agreement in its shape and frequency. Thus this study was performed to clarify the arthroscopic anatomy of the plica, to classify patterns of the normal plica and to discover the incldence of the pathologic plica. The results and conclusions were as follows; 1. From June 1990 to April 1992, 338 knees in 306 persons were examined arthroscopically. 2. We have clarified the arthroscopic anatomy of the synovial folds within the knee joints and classified the synovial plica according to their shape and size on arthroscopic view. 3. Incidences of the synovial plica at the knees are suprapatellar plica, 83.3%; mediopatellar plica, 76.7%; infrapatellar plica, 97.8%; and lateral patellar plica, 2.2%. 4. Incidence of the plica syndrome is 3.70%. 5. No correlation was found in patterns among the four groups of the plica, between men and women, with person,s age, and between right and left knee. 6. No significant disparity was found in patterns for right and left knee in same persons. 7. Many variation in the shape and size of the synovial plica supports the multiple cavitation theory for development of the knee during the fetal stage.