The pyridinium crosslinks are important and definite biomarkers of mature hard tissue collagen degradation. A gradient ion-paired reversed-phase high-performance liquid chromatographic method was used for the simultaneous determination of both crosslinks in synovial fluid (SF) samples of 25 patients with rheumatoid arthritis (RA). The mean +/- SD levels of pyridinoline (Pyd) and deoxypyridinoline (Dpyd) in SF were 107.7 +/- 182.3 nmol/l and 4.8 +/- 8.3 nmol/l, respectively. The Pyd/Dpyd ratio, which indicates the amount of Pyd released from cartilage rather than bone, amounted to 30.8 +/- 29.5. This value is significantly higher than in urine or serum of the same patients. These data suggest increased destruction of joint cartilage in patients with RA and the release of collagen II fragments in SF. In addition, the levels of the crosslinks in SF reflect considerable interindividual variation, indicating substantial individual differences in the amount of collagenous material that is degraded.