Adiponectin is a hormone released by adipose tissue with antidiabetic, antiatherogenic, and anti-inflammatory properties. The present observational study focused on the relation between serum adiponectin level and the disease severity of established rheumatoid arthritis (RA). Ninety patients with more than 5-year diagnosis of RA and 42 age- and BMI-matched control were enrolled. The severity of RA was evaluated according to the number of destructed joints of overall 68 joints on plain radiographs (37 patients had mild RA and 53 had severe RA). Serum adiponectin level was significantly higher in the severe RA group (17.7+/-6.7 microg/ml) than in the control (9.1+/-3.8 microg/ml) and mild RA groups (13.9+/-6.5 microg/ml) (control vs. mild RA group, P<0.001; mild RA vs. severe RA group, P<0.01). These results suggest that increased number of joint destruction is associated with hyperadiponectinemia in established RA patients.