Chemokines are involved in a number of inflammatory pathologies and some of them show a pivotal role in the modulation of osteoclast development. Therefore, we evaluated the role of CXCL12 chemokine on osteoclast differentiation and function and we analyzed its expression on synovial and bone tissue biopsies from rheumatoid arthritis (RA) patients. Osteoclasts were obtained by 7 days in vitro differentiation with RANKL and M‐CSF of CD11b positive cells in the presence or absence of CXCL12. The total number of osteoclast was analyzed by Tartrate‐resistant acid phosphatase (TRAP)‐staining and bone‐resorbing activity was assessed by pit assay. MMP‐9 and TIMP‐1 release was evaluated by ELISA assay. CXCL12 expression on biopsies from RA patients was analyzed by immunohistochemistry. Osteoclasts obtained in the presence of CXCL12 at 10 nM concentration displayed a highly significant increase in bone‐resorbing activity as measured by pit resorption assay, while the total number of mature osteoclasts was not affected. The increased resorption is associated with overexpression of MMP‐9. Immunostaining for CXCL12 on synovial and bone tissue biopsies from both rheumatoid arthritis (RA) and osteoarthritis (OA) samples revealed a strong increase in the expression levels under inflammatory conditions. CXCL12 chemokine showed a clear activating role on mature osteoclast by inducing bone‐resorbing activity and specific MMP‐9 enzymatic release. Moreover, since bone and synovial biopsies from RA patients showed an elevated CXCL12 expression, these findings may provide useful tools for achieving a full elucidation of the complex network that regulates osteoclast function in course of inflammatory diseases. J. Cell. Physiol. 199: 244–251, 2004© 2003 Wiley‐Liss, Inc.