Established osteoporosis in older patients of both sexes is characterized by decoupled bone remodelling induced by sex hormone deficits and by somatopause, but also by lack of vitamin D and reduced synthesis of the D-Hormone (calcitriol; 1.25 (OH)2D) in the kidneys and bone, as well as from lack of receptors and/or receptor affinity for D-Hormone in the target organs. Parallel to the decreased bone strength a loss of muscle power occurs, together with an increase in balance disorders and an increasing risk of "intrinsic", nonsyncopal locomotoric falls. In alfacalcidol therapy, D-Hormone is provided to the body in circumvention of its own regulation, by means of which higher hormone concentrations can be achieved in the target tissues than by administration of plain vitamin D. In vitro and in vivo experiments have provided growing evidence that D-Hormone analogs tend to normalize PTH, lead to an increase in the number and activity of osteoblasts, reduce the activity of osteoclasts, and might thus normalize the "high bone turnover" in elderly osteoporotic patients ("supercoupling"). In addition, it has been shown that D-Hormone analogs are able to increase muscle power and walking distance in elderly D-Hormone deficient patients. Besides the known effect on the vertebral fracture rate, new clinical data confirm that D-Hormone analogs might reduce peripheral fractures by reducing falls. The expanded understanding of the pathogenesis of glucocorticoid- induced osteoporosis with its disturbed calcium homeostasis and the pharmacological effects of alfacalcidol, which counteract such iatrogenic bone loss, contribute to the understanding of its clinical efficacy in this most frequent form of secondary osteoporosis. Due to its recently discovered immunomodulating properties, alfacalcidol might find a slot in the management of bone loss caused by chronic inflammatory diseases or by organ transplantations. Alfacalcidol has multifactorial effects, among which the best known are its anti-bone loss and anti-fracture efficacies in postmenopausal osteoporosis. This demonstrated efficacy is related to its involvement in bone remodelling, leading to an improved bone strength. Its mode of action on muscle power, which reduces falls, is unique, differentiating this form of therapy from all other anti-osteoporotic drugs, none having demonstrated any influence on falls.