INTRODUCTION mt THE INTRODUCIION in 1949") and subsequent widespread use of glucocorticoids as anti-inflammatory and immunosuppressive agents, glucocorticoid-induced osteoporosis (GC-OP) has become a significant clinical problem.(*) This has stimulated considerable interest in the cellular rnechanisms underlying bone loss in GC-OP, and bone histomorphometry has played an important role in beginning to elucidate these. This review describes the histomorphometric profile found in patients with GC-OP, discusses pathogenetic mechanisms, and examines the effects of glucocorticoid withdrawal and of treatments that have been used to combat bone loss. The findings are interpreted in the light of Frost's concept of adult bone rem~ deling.'~) Very briefly, this theory holds that remodeling is a quantum phenomenon in which bone resorption and bone formation are coupled in both space and time. Osteoclasts and osteoblasts work in a coordinated, sequential fashion in bone remodeling units (BRU), which first remove a fixed volume of old bone and then, without interruption, replace it with new. In this system, an agent that stimulates os-teoclastic bone resorption does so by increasing the frequency of activation of BRU and will, under normal circumstances, automatically lead to a coupled increase in bone formation.