OBJECTIVE: To investigate the prevalence of anti-cyclic citrullinated peptide (anti-CCP) and anti-keratin antibodies (AKA) in patients with primary Sjögren's syndrome. METHODS: 149 patients with a diagnosis of primary Sjögren's syndrome according to the European/American consensus criteria were recruited from three French medical centres. The presence of anti-CCP was determined by enzyme linked immunosorbent assay and of AKA antibodies by indirect immunofluorescence. Radiographs of hands and feet were evaluated at the time of anti-CCP analysis. RESULTS: Six patients with radiological erosions and nine patients with non-erosive arthritis fulfilling ACR criteria for rheumatoid arthritis were thought to have rheumatoid arthritis and secondary Sjögren's syndrome, while 134 were considered to have primary Sjögren's syndrome (mean (SD) disease duration, 11.1 (6.6) years). Of these, 80 tested positive for IgM rheumatoid factor (RF) (59%), 10 (7.5%) for anti-CCP, 7 (5.2%) for AKA, and 5 (3.7%) for both anti-CCP and AKA. There was no difference in clinical and biological features, including prevalence of RF, between anti-CCP positive and negative patients. The nine Sjögren patients with non-erosive arthritis, fulfilling ACR criteria for rheumatoid arthritis, were all CCP positive. Their response to disease modifying antirheumatic drugs could be different from classical rheumatoid patients. CONCLUSIONS: Most patients with primary Sjögren's syndrome are negative for AKA and anti-CCP, but positive test results should not rule out this diagnosis. Anti-CCP positive patients, who may be prone to developing rheumatoid arthritis, require cautious clinical and radiographic follow up.