The purpose of the present investigation was to examine and describe the osteoarthritic changes that chronic, partial or complete anterior cruciate ligament (ACL) insufficiency causes to the knee joint. The most characteristic findings in 77 patients with average follow-up of eight years were osteophytes and subchondral sclerosis of femoral and tibial medial condyles, tibial eminence and patella, as well as narrowing of medial or lateral joint space. Only five patients (14%) in the group with partial, but 28 (70%) of those with complete insufficiency of the ACL suffered from a clear posttraumatic osteoarthritis of the injured knee. In complete tears, the total extent of these pathological changes per patient was fourfold compared to the patients with partial tear. It was concluded that chronic, posttraumatic insufficiency of the ACL causes characteristic osteoarthritic changes to the injured knee and that the amount of these changes seems to depend on the amount of the insufficiency. Therefore, a quick restoration of the static and dynamic stability of the injured knee must be the privilege of every knee injury patient in order to prevent the development of premature osteoarthritis.