Quantitative analysis of hyaluronan (hyaluronic acid; HA) in the synovial tissues of patients with joint disorders were performed. HA was found not only in the synovial intimal cells and matrices, but also especially in the alveolar lymphoid follicles and connective tissues surrounding blood vessels in the inflammatory granular synovium which formed the pannus. HA levels in the synovium of patients with rheumatoid arthritis (RA) (459.0+/-66.2 microg/g) where shown to be higher than those in patients with osteoarthritis (246.9+/-34.8 microg/g) and traumatic injury (227.7+/-35.4 microg/g). It follows from the present findings, HA in the synovium might contribute to the high amounts of serum levels of HA in patients with rheumatoid arthritis.