It is uncertain whether the autoantibodies to type II collagen that occur frequently in the serum and synovial fluid of patients with rheumatoid arthritis (RA), but rarely in other articular diseases, are primary or secondary to cartilage damage. Hence, we measured antibodies in synovial fluid from patients with RA and other articular diseases and related these to the concentration of sulphated glycosaminoglycans, as a measure of ongoing cartilage catabolism. Synovial fluids from 42 patients with RA and 30 patients with other articular diseases were studied. We found that levels of antibodies to native and denatured collagen were significantly higher in RA than in all other articular diseases, whereas concentrations of sulphated glycosaminoglycans were similar. The absence of any correlation between levels of sulphated glycosaminoglycans and antibodies to collagen weighs against the occurrence of such antibodies in RA as a secondary effect of cartilage damage.