Values for urinary hydroxypyridinium cross links of collagen measured by high performance liquid chromatography in a group of 19 seropositive patients with rheumatoid arthritis (RA) were compared with 19 controls matched for age and sex. The amounts of pyridinoline, expressed relative to creatinine, were significantly higher in the RA group, but there was no significant increase in a bone specific analogue, deoxypyridinoline. There were significant positive correlations between pyridinoline excretion and both C reactive protein and erythrocyte sedimentation rate, whereas there was an inverse relation with grip strength. The data indicated that pyridinoline concentrations in urine are strongly associated with disease activity in patients with RA.