Information on precise effects of deflazacort on bone cell function, especially osteoclasts, is quite limited. Therefore, the present study was undertaken to test effects of deflazacort on osteoclast-like cell formation in mouse bone marrow cultures and on the regulation of osteoprotegerin (OPG) and its ligand (RANKL) mRNA expressions by RT-PCR in the ST2 marrow stromal cells. TRAP-positive mononuclear cells increased after the treatment of deflazacort at 10^-9 to 10^-7 M alone for 6 days in a dose-dependent manner. Number of TRAP-positive multi-nucleated cells (MNCs) increased significantly with combined treatment of deflazacort at 10^-7 M and 1,25-(OH)₂D₃ at 10^-9 M compared to that of cultures treated with 1,25-(OH)₂D₃ alone (p<0.05). Exposure to deflazacort at 10^-7 M in the presence of 1,25-(OH)₂D₃ at 10^-9 M in the last 3-day culture had greater stimulatory effect on osteoclast-like cell formation than that of the first 3-day culture did. Deflazacort at 10^-10 -10^-6 M downregulated OPG and upregulated RANKL in mRNA levels in a dose-dependent manner. These observations suggest that deflazacort stimulate osteoclast precursor in the absence of 1,25-(OH)₂D₃ and enhance differentiation of osteoclasts in the presence of 1,25-(OH)₂D₃. These effects are, in part, thought to be mediated by the regulation of the expression of OPG and RANKL mRNA in marrow stromal cells.