An early stage of rheumatoid arthritis (RA) has frequently an atypical course and it is difficult to differentiate it from other types of polyarthritis. It has been proved, that anti-cyclic citrullinated peptide antibodies (anti-CCP) are highly specific marker for RA. Anti-CCP can be found several years before clinical diagnosis of RA. The aim of the study was a retrospective assessment of usefulness of anti-CCP antibodies in differential diagnosis of polyarthritis. We analyzed case histories of 320 patients (pts) with symptoms of polyarthritis, hospitalized in the Department of Rheumatology and Connective Tissue Diseases between 2003 and 2004. The presence of anti-CCP was determined in all patients by enzyme linked immunosorbent assay (ELISA). The positive measurement was obtained in 165 pts. Further analysis was performed in 124 pts with significantly (20-80 RU/ml) and highly (>80 RU/ml) positive measurements of antibodies. During two years of observation, RA was diagnosed (according to the American College of Rheumatology, ACR, criteria) in 97 pts. A therapy with disease modifying antirheumatic drugs was administered in all the patients. The remaining patients did not fulfill the ACR criteria for RA in our observation period. The following diagnoses were established: in 9 pts other connective tissue diseases (4 pts with undifferential connective tissue disease – UCTD, 3 pts with systemic lupus erythematosus – SLE, 1 pt with primary Sjögren’s syndrome, 1 pt with scleroderma), in 1 pt systemic small-vessel vasculitis, in 12 pts undifferential arthritis (UA), in 1 pt psoriatic arthritis, in 4 pts osteoarthritis. All the patients with diagnosis of UA and UCTD will be still observed for the development of RA. It can be concluded that determination of anti-CCP antibodies in patients with polyarthritis allows us to confirm the diagnosis of RA and start treatment with disease modifying anti-rheumatic drugs. The patients with non-RA diagnoses should be carefully observed for the development of RA in the future.