The authors have performed 1 005 arthroscopies of the knee for diagnostic purposes. The exploratory method has made it possible to correct the initial diagnosis in 50% of the cases, to avoid arthrotomy in 10% and to suggest surgery in 9%. The most common cause of error was the radioclinical confusion between patellar and medial meniscus symptoms. The most frequent associated lesions were patellar chondropathy (73%), condylar chondropathy (33%) and middle patellar folds (34%).