There may be a great potential in the use of diagnostic "markers" of osteoarthritis in synovial fluid to diagnose the disease in an earlier stage and perhaps assess the severity of the disease and monitor the effect of a treatment. In the present study, potential markers are characterized, discussed, and grouped according to the latest knowledge on the etiology, pathogenesis, and pathology of osteoarthritis. They are grouped according to their origin as either cartilage degradation products, or related to the mechanisms of cartilage degradation, or related to chrondrocytic anabolic activity during disease, or related to genetic disorders. Also potential markers that have not yet been studied clinically or experimentally are discussed. Examples of the progress that has been made in human medicine approaching reliable diagnostic markers that should also be tried in veterinary medicine are described.