The anatomy of the medial suprapatellar plica and medial synovial shelf was studied arthroscopically in 500 knees. The medical suprapatellar plica extended up to one third of the way across the suprapatellar pouch in 64.2% of knees, between one and two thirds in 4%, and two thirds or more in 31.6%. The medial synovial shelf was absent or vestigial in 36% of knees and broader than 1 cm in 13.2%. When both knees were examined arthroscopically, the similarity between the appearances of the plica and the shelf in the two knees was statistically significant. No relationship between age and the pattern of plica or shelf could be found. There was no association between large plicae and large shelves, but the shelf was absent significantly more often in knees with a narrow plica.