The synovial plicae which were removed were at first recognized as abnormal structures in the knee. After Mayeda's description and Iino's investigation about the anatomy of the synovial folds within the knee, they are considered as normal structures today. But the variations of their shape and the frequency with which they occur are not agreed upon. The medial synovial shelf runs in the cornal plane into the infrapatellar fat pad. Iino reported the shelf to be found in more than 50% of the knees, Aoki in 20%, Jackson in 60%, and Satoshi in 37%, Irritation of the medial synovial shelf may give rise to symptoms. We attempted to represent the antomy of the suprapatellar plica and the medial synovial shelf including their incidence and variations in shape, which is extemely variabl, within Korean knees. In the suprapatellar plica, it was found that 90.2% of knees had some evidence of the plica such as complete membranes, arches, perforated membrance, and pillar shapes. In the medial synovial shelf it was found that 80% of knees had smoe evidence of the shelf such as vestigial shelf, medium shelf, large shelf, perforated shelf and redupulicated type according to sakakibara classification modified by the authors. Pathologic shelves were 3.7%. The suprapatellar plica and the medial synovial shelf were found more frequently than ever reported in Korean knees. The anatomy of the suprapatellar plica and medial synovial shelf was investigated arthroscopically in 132 knees between July 1990 and Jun. 1991. The suprapatellar plica were found in 90.2%, the medial synovial shelf in 80% and the lateral synovial shelf in 1%. There was no association between large plicae and large shelves.