I n agreement with evidence that estrogens slow the rate of bone remodeling by suppressing the production of both osteoclasts and osteoblasts, loss of estrogens leads to an increase in the number of osteoclast as well as early osteoblast progenitors (CFU-osteoblasts; CFU-OBs) in the murine bone marrow. Here we show that CFU-OBs are early transit-amplifying progenitors, i.e., dividing cells capable of limited self-renewal, and that 17β-estradiol acts in vivo and in vitro to attenuate their self-renewal by approximately 50%. Consistent with a direct receptor–mediated action of estrogens on early mesenchymal cell progenitors, anti–estrogen receptor-α (anti-ERα) Ab’s stain a small number of marrow cells that exhibit characteristics of primitive undifferentiated cells, including a high nucleus/cytoplasm ratio and lack of lineage-specific biochemical markers; the effect of 17β-estradiol on CFU-OB self-renewal is absent in mice lacking ERα. Because both osteoblasts and the stromal/osteoblastic cells that are required for osteoclast development are derived from CFU-OBs, suppression of the self-renewal of this common progenitor may represent a key mechanism of the anti-remodeling effects of estrogens.