Background and aim. Postmenopausal osteoporosis, caused by estrogen deficiency, is characterized by the structural deterioration of bone accompanied by an increase in bone marrow adipocytes. Transgenic animal models have shown that there is a reciprocal relationship between osteoblastogenesis and adipogenesis in vivo. The present study investigated whether the estrogen and the canonical Wnt signaling pathways are linked together and regulate the phenotype and function, differentiation and proliferation of human osteoblasts using an in vitro estrogen-deficiency model.Methods. Human osteoblasts (hFOB 1.19) and fulvestrant an estrogen receptor blocker, were used to mimic estrogen deficiency in vitro. Osteogenic and adipogenic differentiation was measured by using specific stains and microscopy, as well as by measuring the expression of bone cell-specific markers with reverse transcription polymerase chain reaction. Expression of estrogen receptor-alpha (ER alpha) and beta-catenin was detected in Western blots and by immunoprecipitation.Results. The cells expressed the 46-kDa and the 77-kDa ER alpha isoforms and beta-catenin. Fulvestrant reduced expression of ER alpha and beta-catenin beta-Catenin was co-immunoprecipitated with ER alpha, indicating that these two proteins form a new signaling complex and transcription factor. In addition, it induced intracellular lipid droplet accumulation and downregulation of bone cell markers, indicating adipocyte differentiation.