The aim of this study was to determine whether the inflammatory milieu and/or hypoxia induces the dedifferentiation of synovial cells into mesenchymal stem-like cells, which may contribute to the tumor-like growth of synovial cells. Expression of mesenchymal stem cell markers (CD24, CD44, CD90, CD106, CD146 and Stro-1) was compared among cultured fibroblast-like synoviocytes (FLSs) from patients with rheumatoid arthritis (RA) or osteoarthritis (OA), bone marrow mesenchymal stem cells (BM MSCs) and normal dermal fibroblasts. After the cells were stimulated with pro-inflammatory cytokines for 3 days under hypoxia or normoxia, the stem cell markers were analyzed by FACS. CD44 and CD90 were expressed constitutively in all four cell types. Only the BM MSCs strongly expressed CD146. The expression of stem cell markers was similar between FLSs from RA and those from OA patients. In addition, the expression levels in FLSs were similar to those in normal dermal fibroblasts. The stimulation of FLSs and dermal fibroblasts with IL-1β or a mixture of cytokines under hypoxia did not induce a marked change in the expression of stem cell markers. These results indirectly suggest that the pro-inflammatory milieu may be not sufficient to induce the dedifferentiation of FLSs in arthritic joints.