Summary The receptor activator of nuclear factor kappa B ligand (RANKL) and its decoy receptor, osteoprotegerin (OPG), are important bone metabolism molecules, which directly control osteoclastogenesis. Periodontal ligament (PDL) cells play a vital role in maintaining the homeostasis of periodontal tissues, releasing cytokines to affect bone metabolism. The purpose of this study was to investigate the expression of OPG and RANKL in cultured human periodontal ligament cells (hPDLCs) derived from permanent teeth and the expression change after stimulation by 1a, 25-dihydroxyvitamin D3 (1a,25(OH)2vitD3), a kind of bone resorption promoter. HPDLCs were cultured in the presence or absence of 10 8 M 1a,25(OH)2vitD3 in vitro. The expression of mRNA for OPG and RANKL in hPDLCs during 6 days'culture was examined using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). The level of secreted OPG protein in the culture medium during 6 days'culture was detected by enzyme-linked immunoabsorbent assay (ELISA). The result showed that OPG and RANKL were expressed by hPDLCs. OPG expression was downregulated by 10 8M 1a,25(OH)2vitD3 in a time-dependent manner, while RANKL mRNA was up-regulated. The ratio of OPG/RANKL was decreased. In conclusion, our findings suggest that hPDLCs may regulate the alveolar bone metabolism through the OPG/RANKL system.