Recent clinical studies suggest that several antihypertensive drugs, especially angiotensin-converting enzyme inhibitors, reduced bone fractures. To clarify the relationship between hypertension and osteoporosis, we focused on the role of angiotensin II (Ang II) on bone metabolism. In bone marrow-derived mononuclear cells, Ang II (1x10(-6) M) significantly increased tartrate-resistant acid phosphatase (TRAP) -positive multinuclear osteoclasts. Of importance, Ang II significantly induced the expression of receptor activator of NF-kappaB ligand (RANKL) in osteoblasts, leading to the activation of osteoclasts, whereas these effects were completely blocked by an Ang II type 1 receptor blockade (olmesartan) and mitogen-activated protein kinase kinase inhibitors. In a rat ovariectomy model of estrogen deficiency, administration of Ang II (200 ng/kg/min) accelerated the increase in TRAP activity, accompanied by a significant decrease in bone density and an increase in urinary deoxypyridinoline. In hypertensive rats, treatment with olmesartan attenuated the ovariectomy-induced decrease in bone density and increase in TRAP activity and urinary deoxypyridinoline. Furthermore, in wild-type mice ovariectomy with five-sixths nephrectomy decreased bone volume by microcomputed tomography, whereas these change was not detect in Ang II type 1a receptor-deficient mice. Overall, Ang II accelerates osteoporosis by activating osteoclasts via RANKL induction. Blockade of Ang II might become a novel therapeutic approach to prevent osteoporosis in hypertensive patients.