Background Bone damage around the joints is one of the major pathophysiological mechanisms that leads to rheumatoid arthritis (RA) chronic disability.Serum tartrate-resistant acid phosphatase 5b (TRACP-5b) is secreted by osteoclasts,its activity can be used as a clinically relevant bone resorption marker.The aim of this study was to test whether the measurement of serum levels of TRACP-5b in patients with RA would correlate with measures of disease activity and with responses to therapy.Methods Fifty-six patients were randomly assigned to receive recombinant human cytotoxic tlymphocyte-associated antigen-4 immunoglobulin (RhCTLA4-lg),infliximab or methotrexate (MTX).The clinical and serologic indicators of RA activity were evaluated at baseline and at 24 weeks.Serum TRACP-5b was measured by Enzyme-linked Immunosorbent Assay (ELISA) at 0,12 and 24 weeks.Hand X-rays were obtained at baseline.Results At baseline,the levels of TRACP-5b correlated with the severity of X-ray damage,disease duration (r=0.332,P=0.012),and tender joint count (r=0.408,P=0.002).The 24 weeks values of TRACP-5b for RhCTLA4-lg group and infliximab group differed significantly from the baseline values in each group (P ＜0.05; P ＜0.05),whereas only the value for RhCTLA4-lg group differed significantly from the 24 weeks value for the MTX group (P ＜0.01).Considering the two biologics-treated groups together,the TRACP-5b levels at 24 weeks differed significantly from the baseline values only in those patients who reached an ACR70 level (P ＜0.05).Conclusions Measurement of serum TRACP-5b in RA patients reflects clinical and radiological measures of disease activity,treatment with certain biologics,and degree of response to therapy.TRACP-5b should be investigated further as a potential biomarker to predict response to therapy,including slowing of radiographic progression.