Osteoprotegerin (OPG), a tumor necrosis factor (TNF) receptor family member, is a critical regulator of bone resorption. It is an important inhibitor of the terminal differentiation and activation of osteoclasts. This randomized, double-blind, placebo-controlled, sequential dose escalation study was conducted in postmenopausal women to determine the effect of a single subcutaneous (s.c.) dose of OPG on bone resorption as indicated by the biochemical markers, urinary N-telopeptide (NTX) and deoxypyridinoline (DPD), which are stable collagen degradation products. NTX levels decreased within 12 h after OPG administration. At the highest dose administered (3.0 mg/kg), a mean percent decrease in NTX of approximately 80% was observed 4 days after dosing. Six weeks after dosing a mean decrease of 14% in NTX was observed. The levels of bone-specific alkaline phosphatase (BSAP), a marker of bone formation, did not change for approximately 3 weeks after dosing. Thereafter, a modest decrease, reaching approximately 30% at 6 weeks, was observed in the 3.0-mg/kg dose group. The rapid decrease from baseline in NTX and delayed decrease in BSAP indicated that OPG acted primarily on osteoclasts to decrease bone resorption. OPG injections are well tolerated. This study, for the first time, indicates that a single s.c. injection of OPG is effective in rapidly and profoundly reducing bone turnover for a sustained period and that OPG therefore may be effective in treatment of bone diseases characterized by increased bone resorption such as osteoporosis.