BACKGROUND We have demonstrated that a peptide, which we named 'Peptide A', derived from the extracellular domain of T-cell leukemia translocation-associated gene (TCTA) protein, inhibited human osteoclastogenesis. OBJECTIVE In the current study, we examined whether this peptide inhibits the proliferation of rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS) or not. MATERIAL AND METHODS Fibroblast-like synoviocytes obtained from five RA patients were cultured in the absence or presence of 1, 5, 10 µg/ml of peptide. We used 29-mer scrambled peptide as a control. RESULTS The peptide inhibited the proliferation of RA FLS dose-dependently. On the other hand, the scrambled peptide showed no inhibition. CONCLUSIONS The peptide inhibits both human osteoclastogenesis and the proliferation of RA FLS. Thus, the peptide may be used for the therapy of both osteoporosis and synovitis of RA patients. This is the first report of the new peptide we discovered, which inhibits both osteoclastogenesis and synovitis. Thus, this new peptide could be a new drug for patients with both osteoporosis and RA.