The present study determined the levels in synovial fluid (SF) of vitamin D metabolites (1,25-dihydroxyvitamin D (1,25(OH)(2)D), 24,25-dihydroxyvitamin D (24,25(OH)(2)D) and 25-hydroxyvitamin D (25-OH-D)), and of the cytokines. We evaluated SF from 21 patients with rheumatoid arthritis (RA) and 6 patients with osteoarthritis (OA). The levels of vitamin D metabolites in SF, as determined by two different extraction methods, were significantly correlated (p < 0.05, n=7). The levels of 3 vitamin D metabolites were significantly higher in the RA SF than in OA SF (p < 0.05). The ratio of 1,25(OH)(2)D/25-OH-D in RA SF, which is presumed to reflect the activity of 25-OH-D-1-hydroxylase (1-OH-ase), was positively correlated with the levels of interleukin-1 alpha (IL-1 alpha), IL-1 beta, and IL-2 in such SF, and was significantly higher than that in sera from RA patients. This suggests an important role for these cytokines in the activation of 1-OH-ase in RA synovium. The ratio of 24,25(OH)(2)D/25-OH-D, which is presumed to reflect 25-OH-D-24-hydroxylase (24-OH-ase) activity, was significantly correlated with 1,25(OH)(2)D levels only in RA SF, but not in sera from RA patients, suggesting a local regulation of vitamin D metabolism that 1,25-(OH)(2)D induces 24-OH-ase as in other target cells. Our observations suggested that 1,25(OH)(2)D and 24,25(OH)(?)2D are produced locally from 25-OH-D in RA synovium, and that the syntheses of 1,25(OH)(2)D and 24,25(OH)(2)D may be affected by IL-1/IL-2 and 1,25(OH)(2)D in RA SF, respectively.