The role of Cbfa1 (core binding factor α1), an essential transcriptional factor for osteoblast differentiation, in osteoclastogenesis was investigatedin vitroandin vivousingCbfa1-deficient calvarial cells and mice. Co-cultures of calvarial cells isolated from embryos with three differentCbfa1genotypes (Cbfa1 + / + ,Cbfa1 + / - andCbfa1 - / - ) and normal spleen cells generated TRAP-positive multinucleated osteoclast-like cells (OCLs) in response to 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2 D 3 ] and dexamethasone, but the number and bone-resorbing activity of OCLs formed in co-culture withCbfa1 - / - calvarial cells were significantly decreased in comparison with those formed in co-cultures withCbfa1 + / + orCbfa1 + / - calvarial cells. The expression ofosteoclast differentiation factor/osteoprotegerin ligand(ODF/OPGL) mRNA was increased by the treatment with 1α,25(OH) 2 D 3 and dexamethasone in calvarial cells fromCbfa1 + / + andCbfa1 + / - mouse embryos, but not fromCbfa1 - / - embryos. In contrast, the expression ofosteoprotegerin/osteoclastogenesis inhibitory factor(OPG/OCIF) mRNA was inhibited by 1α,25(OH) 2 D 3 and dexamethasone similarly in all three types of calvarial cells.ODF/OPGLandOPG/OCIFmRNAs were highly expressed in the tibia and femur ofCbfa1 + / + andCbfa1 + / - embryos. In the tibia and femur ofCbfa1 - / - embryos, however,ODF/OPGLmRNA was undetectable and the expression ofOPG/OCIFmRNA was also decreased compared with those inCbfa1 + / + andCbfa1 + / - embryos. These results suggested that Cbfa1 is somehow involved in osteoclastogenesis through regulation of ODF/OPGL.