In a retrospective study of patients presenting with symptoms of knee instability, 16 patients were discovered who had developed severe chondromalacia or osteoarthritis of the femoral condyles. All of the patients gave a history of a previous severe knee injury occurring in their teenage years and all had continued to play competitive sport. Ten of the group had subsequent meniscal injuries requiring surgery. In nine of the group previous arthroscopic or open joint procedures had demonstrated normal femoral joint cartilage. There was no difference in the degree of degeneration in those who had had a meniscectomy as compared with those who had not had meniscal damage. It is concluded that severe anterolateral instability is a cause of early degenerative joint disease in young athletes even in the absence of meniscal damage and that they should be strongly advised against participating in active sport until the joint has been stabilized.