Adiponectin belongs to the family of adipokines, the proteins produced by the white adipose tissue, which is presently considered as an active secretion organ. Adiponectin exerts a protective effect on blood vessels and is claimed to play a preventive role in obesity. Adiponectin&#8217;s function in inflammation and rheumatic disorders has not been clarified to date, but the protein is considered an important inflammatory modulator. In this study we assessed serum adiponectin levels in rheumatoid arthritis (RA) and osteoarthritis (OA) patients and found a correlation between serum adiponectin level and other markers as well as bone mass density changes, activity of disease, disease duration and the age of the patients. The blood was collected from 44 RA and 42 OA patients who constituted the control group. The serum level of adiponectin in RA and OA patients ranged respectively from 2.6 to 50 &#181;g/ml (median 14.91) and 4.6 to 39.64 &#181;g/ml (median 10.07). No differences in adiponectin concentrations were noted between RA and OA patients, nor between subsets of different age or body mass in both groups. A negative correlation between adiponectin concentrations and body mineral density (BMD) was found in both RA (r = &#8211;0.35; p < 0.05) and OA groups (r = &#8211;0.42; p < 0.05). In osteoarthritis higher adiponectin concentrations were noted in longer-lasting disease (r = 0.43; p < 0.05).