Serum levels of pentosidine, an advanced glycation end product that is increased in rheumatoid arthritis (RA), were measured in serum, synovial fluid or urine in 20 patients with rheumatoid arthritis (RA) and compared with 22 patients with osteoarthritis (OA), and the relationship between pentosidine and RA disease activity was evaluated. A significant correlation between penosidine levels in serum, synovial fluid and urine was found, and an association of the first two with the systemic inflammatory activity of RA. Moreover, increased serum levels do not derive from synovial fluid but from the whole body in RA. They conclude that pentosidine is a superior indicator for RA activity.