The receptor activator of NF-B (RANKL) is the essential signal required for full osteoclast (OC) development, activation, and survival. RANKL is highly expressed in areas of trabecular bone remodeling and inflammatory bone loss, is increased on marrow stromal cells or osteoblasts by osteotropic hormones or cytokines, and is neutralized by osteoprotegerin (OPG), a soluble decoy receptor also crucial for preventing arterial calcification. Vascular endothelial cells (VEC) are critically involved in bone development and remodeling and influence OC recruitment, formation, and activity. Although OCs develop and function in close association with bone VEC and sinusoids, signals mediating their interactions are not well known. Here, we show for the first time that human microvascular endothelial cells (HMVEC) express transcripts for both RANKL and OPG; inflammatory cytokines tumor necrosis factor-␣ and interleukin-1␣ elevate RANKL and OPG expression 5-40-fold in HMVEC (with an early OPG peak that declines as RANKL rises), and RANKL protein increases on the surface of tumor necrosis factor-␣-activated HMVEC. Cytokine-activated HMVEC promoted the formation, fusion, and bone resorption of OCs formed in co-cultures with circulating human monocytic precursors via a RANKLmediated mechanism fully antagonized by exogenous OPG. Furthermore, paraffin sections of human osteoporotic fractured bone exhibited increased RANKL immunostaining in vivo on VEC located near resorbing OCs in regions undergoing active bone turnover. Therefore, cytokine-activated VEC may contribute to inflammatorymediated bone loss via regulated production of RANKL and OPG. VEC-derived OPG may also serve as an autocrine signal to inhibit blood vessel calcification.The receptor activator of NF-B ligand (RANKL), 1 also known as osteoprotegerin ligand (OPGL), osteoclast differentiation factor, or TNF-related activation-induced cytokine (TRANCE), is a recently discovered transmembrane molecule of the tumor necrosis factor (TNF) ligand superfamily that is highly expressed in lymphoid tissues and trabecular bone, particularly in areas associated with active bone remodeling or inflammatory osteolysis (1-4). RANKL is the essential and final common signal required both in vitro and in vivo for full osteoclastic (OC) differentiation from multipotential hematopoietic precursor cells into mature multinucleated bone-resorptive OCs in the presence of the permissive factor macrophage colony-stimulating factor (M-CSF) (1-7). RANKL expressed on the surface of osteoblasts (OB) or bone marrow stromal cells (BMSC) interacts with a cell surface receptor, RANK, present on pre-OC (induced by M-CSF) and mature OC to stimulate their fusion, development, bone resorption, and cell survival (5-9). RANKL expression increases during early OB development and is up-regulated in OB and BMSC by various proresorptive stimuli such as parathyroid hormone (PTH), 1,25-dihydroxyvitamin D 3 (VD 3 ), dexamethasone (Dex), prostaglandin E 2 , or interleukin-11 (IL-11) (6, 10 -12). Recently, the pro-reso...