Objectives: Anti-carbamylated protein antibodies (anti-CarP) are reported to be associated with increased disease activity and with more severe joint damage in rheumatoid arthritis (RA) patients. The present study investigated the presence of anti-CarP in various rheumatic diseases, and their specific clinical significance in RA, in Belgian rheumatology patients.Method: We tested sera from 254 RA patients, 56 healthy controls, and 153 patients with different rheumatic conditions: juvenile idiopathic arthritis (JIA), axial spondyloarthritis, systemic sclerosis, and Sjögren's syndrome (SS). An in-house enzyme-linked immunosorbent assay was used to detect immunoglobulin G antibodies against carbamylated foetal calf serum.Results: Anti-CarP were detected in 88 RA patients (34.6%), of whom 82% were also positive for anti-citrullinated protein antibodies (ACPAs) and 81% were also rheumatoid factor (RF) positive. Of note, 11 anti-CarP single-positive patients were detected (4.3%). The previously reported association with joint erosions was not detected. However, in ACPA- and RF-negative RA patients, the presence of anti-CarP was associated with higher disease activity and disability. Fifteen per cent of JIA patients and 30% of SS patients also tested positive for anti-CarP and their antibody levels did not differ significantly from those of anti-CarP-positive RA patients. Anti-CarP levels were, however, significantly higher in ACPA- or RF-positive patients.Conclusion: Anti-CarP antibodies were detected in the sera of a cohort of Belgian RA patients. Moreover, they were also detected in primary SS patients and in JIA patients. In the seronegative subset of RA patients, anti-CarP antibodies showed prognostic value.