"The skeletal resistance to PTH" which is receiving much attentions especially after the break through in the field of osteoclastogenesis/activation research, is a practical pathogenesis of adynamic bone diseases. Serum osteoprotegerin (OPG) levels increase along with the deteriorations of glomerular filtration rate, and that in dialysis patients would be high enough to demonstrate a osteoclastogenesis inhibitory action in vitro. In fact, the parameters for bone resorption in biopsied iliac bone from dialysis patients showed a negative correlation with serum OPG levels. Moreover, serum OPG concentration increases with aging even among dialysis patients. Therefore "the skeletal resistance to PTH" in aged dialysis patients should be greater than that in younger ones, that possibly explains why adynamic bone disease is more common in aged patients. Thus, circulating OPG in uremic patients may promote the development of adynamic bone disease through the osteoclastogenesis inhibitory action.