It is proposed that involutional osteoporosis could be subdivided into two distinct syndromes—type I osteoporosis and type II osteoporosis—that differed with respect to changes in regional bone mineral density (BMD), pattern of fractures, hormonal changes, and causal mechanisms. This chapter reviews this model, and updates and modifies it based on new observations, especially the findings that estrogen deficiency is a major cause of bone loss in elderly women and, perhaps also, in aging men. The type I/type II model of involutional osteoporosis is supported by four different types of data: differences in fracture patterns as assessed by epidemiologic studies, patterns of bone loss as assessed by bone densitometry, differences in parathyroid function, and differences in hormonal mechanisms of bone loss. That these data are largely independent of each other and yet lead to the same conclusion is strongly supportive of the central hypothesis. For any hypothesis to be useful, it should lead to predictions that can be tested experimentally. The limited number of tests that have been made thus far have all supported the hypothesis and are reviewed in this discussion.
