The authors report on synovial folds of the knee examined by arthroscopy. 60 arthroscopies in 59 patients showed a plica synoviale. The authors recount the different anatomical types. A suprapatellar plica is found in all the cases and a lateropatellar plica is found in 7 out of 60 cases. The suprapatellar plica forms a fibrous fold which is sometimes completely folded off (6.6%) but more often it is an incomplete fold: internal (66.6%), external (21.6%) or annular (15%). In some cases arthroscopy reveals cartilaginous lesions associated with the plica. 18 patients out of 37 showed lesions of secondary chondromalacia. Endoscopic resection offers a possibility of treatment for this condition. The results of this operation were good or excellent in 64.7% of the patients without cartilaginous complications and in 40% of the patients who had a lesion of their cartilages. In the discussion the authors present the possible physiopathology of this syndrome and the presenting symptoms.