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Background. Anti-cyclic citrullinated peptide antibodies (ACPA) are a helpful biomarker in the diagnosis of rheumatoid arthritis (RA) and their presence is predicting erosive disease. However, the correlation between the serum level of ACPA and disease activity in RA is still a matter of controverse. Objective. To analyze the hypothesis that quantitative ACPA would bring complementary information with regard to disease activity in the follow up of patients diagnosed with definite RA. Patients and methods. In a prospective observational study, we followed up for 12 months a RA patient group in whom anti-TNFα biologic treatment was initiated. A group of RA patients treated with classical DMARDs was followed up as a control group. Quantitative ACPA assessment has been performed in all patients at baseline and at 6 and 12 months of follow-up. Results. Patients receiving biologic agents have been more frequently ACPA-positive than those on synthetic DMARDs. ACPA-positive patients has higher disease activity at baseline when compared to ACPA-negative patients (DAS28-ESR with 4 variables: 6,17±0,78 vs. 5,16±0,65, p < 0,001 by independent samples t test). Both in biologicand sznthetic DMARDs patient groups, the serum levels of ACPA and of the rheumatoid factor (RF) decreased significantly at 6 and 12 months. Pooling together the two patient groups (n = 236), the median ACPA titer decreased from 87 U/ml (at baseline) to 53 U/ml (at 12 months). We analyzed by Pearson and Spearman correlations the relationship between the ACPA serum level and the DAS28 score at 0, 6 and 12 months, as well as the relation between the ACPA level variation and the DAS28 variation between the 6- and respectively 12-month visits and baseline. Regardsless of treatment, ACPA levels were not significantly associated to disease activity. Conclusion. Although ACPA levels tend to decrease together with the disease activity score, the lack of correlation between these parameters suggest that repeating the quantitative ACPA test during treatment brings no demonstrable benefit. [ABSTRACT FROM AUTHOR]