The purpose was to identify the effects of estrogen on the expression of alkaline phosphatase (ALP) and osteoprotegerin (OPG) in human periodontal ligament cells (hPDLCs). Cells were obtained from healthy periodontal tissue explants of teeth that had been extracted for orthodontic reasons. ALP activity was determined after 17ß-estradiol (17ß-E2) treatment at 0, 0.1, 1, 10, and 100 nmol/L by using assay reagents. OPG expression in hPDLCs was studied by enzyme-linked immunosorbent assay and reverse-transcriptase polymerase chain reaction. It was shown that the ALP activity and the OPG protein production were increased in a time-dependent manner (P .05). Whereas a physiologic concentration of 17ß-E2 enhanced the secretion of OPG (P .05) and the expression of OPG mRNA, a high dose (100 nmol/L) of 17ß-E2 had no such effect on hPDLCs. It appears that estrogen may exert its antiresorptive effects and maintain periodontal health through stimulating ALP and OPG expression in hPDLCs.