Objectives: To investigate the effect of 17 beta-estradiol (E2) on osteoclastogenesis induced by human periodontal ligament fibroblasts (PDLFs) and by human gingival fibroblasts (GFs). Methods: Confluent layers of PDLFs and GFs were cocultured with human peripheral blood mononuclear cells (PBMCs) in DMEM supplemented with 10-7 M 1α, 25-dihydroxyvitamin D3 and 10-8 M dexamethasone in the presence or absence of various concentration of estradiol E2 including 10-11, 10-9, and 10-7 M E2 for 14 days. Image analysis was used to assess cell-free areas and the number of TRAP-positive multinucleated osteoclast-like cells. Results: PBMCs seeded on the cultured fibroblasts resulted in withdrawal of the latter cells and thus induced the formation of cell-free areas; areas in which osteoclastogenesis occurred. PBMCs induced significantly more cell-free areas in PDLF than in GF cultures. Concomitantly the number of TRAP-positive multinucleated cells proved to be significantly higher in PDLF cultures compared to GF cultures. E2 inhibited both the formation of cell-free areas and the occurrence of TRAP-positive multinucleated cells in a dose-dependent manner (p<0.05). Conclusions: PDLFs induce higher numbers of osteoclast-like cells than GFs and estradiol inhibits this process. Our data suggest that these two subsets of fibroblasts may play different roles in the formation of osteoclasts and that estradiol modulates osteoclastogenesis. Supported by PSU grant 02/2546.