Abstract The fucosylation of haptoglobins is altered in rheumatoid arthritis. In order to investigate the clinical usefulness of this finding, serum levels of abnormallyfucosylated haptoglobins (FHp) have been assessed in defined and matched groups of patients with different inflammatory joint diseases. FHp was elevated in 16 17 patients with active rheumatoid arthritis (RA); 1 20 patients with inactive rheumatoid arthritis; 1 11 patients with osteoarthritis; and 4 10 patients with seronegative polyarthritis. Raised FHp levels, therefore, are not disease-specific. There was no relationship between the duration of RA and the FHp level. The FHp expression in RA was also compared with other biochemical indices of disease activity. The degree of correlation between FHp and articular index, joint score and early-morning stiffness was very similar to that obtained for C reactive protein (CRP), and better than that obtained for erythrocyte sedimentation rate and haemoglobin. FHp, however, gives fewer false-positives than CRP in cases of inactive disease. Until FHp can be measured more easily and cheaply, CRP estimation is still the biochemical test of choice in RA.