Considerable evidence supports the hypothesis that estrogen prevents bone loss by blocking the bone marrow cell production of pro-osteoclastogenic cytokines. However, controversy remains on the role of candidate factors, such as tumor necrosis factor (TNF) and interleukin-6 (IL-6). To investigate the contribution of these cytokines to the pathogenesis of ovariectomy (OVX)-induced bone loss, OVX mice were treated with either TNF binding protein (TNFbp), an inhibitor of TNF, the anti-(IL-6) antibody (Ab) 20F3, or estrogen for the first 2 weeks after surgery. OVX caused a rapid decrease in trabecular bone volume (TBV) and an increase in in vivo bone resorption, as assessed by bone histomorphometry. Treatment with TNFbp completely prevented bone loss and the increase in both osteoclast formation and bone resorption induced by OVX, but had no effects in sham-operated controls. In contrast, treatment with anti-IL-6 antibody failed to prevent bone loss, and the increase in bone resorption and osteoclastogenesis induced by OVX. These data demonstrate that in nongenetically manipulated mice, the estrogen-regulated cytokine that plays a central role in the mechanism by which estrogen deficiency causes bone loss is not IL-6, but rather TNF.