Synovial fluids from the knees of patients with degenerative joint disease (n = 29), osteoarthritis (n = 16), diabetic arthropathy (n = 12), gout (n = 7) and acute inflammatory joint disease (n = 7) were investigated by high-performance size-exclusion chromatography combined with multiangle laser light scattering detection and differential refractometry. These data were compared with the viscosities of the same samples measured by rotation viscometry with one low shear rate, as well as with C reactive protein. The median value of the weight-average molecular weight of hyaluronan in synovial fluids, which differed less than the viscosity of these groups, varied between 1.09 x 10(6) g/mol (range 0.849-1.63 x 10(6) g/mol) (acute-inflammatory joint disease) and 1.91 x 10(6) g/mol (range 1.06-3.48 x 10(6) g/mol) (degenerative joint disease). The correlation between viscosity and hyaluronan concentration was much better than between viscosity and weight-average molecular weight. Changes in C reactive protein concentration were correlated with the disease activity. The concentration of hyaluronan was significantly higher in the cases of degenerative joint disease and diabetic arthropathy. These results suggest that synovial fluid concentration of hyaluronan is appropriate as a prognostic value in the evaluation of different kinds of joint diseases.