The metastasis-associated protein S100A4 is a member of a large family of calcium-binding proteins that appears to play regulatory roles in cancer promoting activities. S100A4 protein regulates cell motility, proliferation, apoptosis, and stimulation of angiogenesis as well as remodeling of the extracellular matrix. It has been demonstrated that proliferating synovial fibroblasts in rheumatoid arthritis (RA) up-regulate expression of S100A4 mRNA. We have previously shown increased expression of S100A4 protein at the sites of joint destruction in RA synovial tissue. The aim of our current study was to examine the involvement of S100A4 in the pathogenesis of RA. Immunohistochemical and immunofluoresence studies were performed to analyze cell-specific distribution of S100A4 protein in RA and osteoarthritis (OA) synovial tissues. Western blotting was used to quantify the activation of the tumor suppressor protein p53 in synovial fibroblasts. Plasma concentrations of S100A4 from RA and OA patients were measured by sandwich ELISA. As we previously shown that S100A4 oligomer is involved in the regulation of several matrix degrading enzymes, in this study we could demonstrate that it can also modulate the transcriptional activation function of the tumor suppressor protein p53 in RA synovial fibroblasts. Moreover, we have identified increased expression of S100A4 in RA compared to OA synovial tissue and several S100A4 producing cells such as synovial fibroblasts, macrophages, mast cells, neutrophils, fraction of T-cells, endothelial cells and pericytes, but not B-cells, within the synovium. The local up-regulation of S100A4 was accompanied by high plasma and synovial fluid concentrations of the S100A4 protein in RA patients. Unlike other S100 proteins including S100A8/9 and S100A12, S100A4 plasma concentrations were not modified by TNF blocking therapy in active RA patients. Taken together, it can be speculated that increased S100A4 protein in circulation and locally at sites of inflammation, particularly at sites of joint destruction, might be implicated in the process of aggressive fibroblast behavior contributing to the pathogenesis of rheumatoid arthritis.