This article suggests classifying "osteoporoses" by their biomechanical pathogenesis instead of by their severity or their accompanying medical conditions. (A) In a "true osteoporosis," bone fragility would increase to such an extent that normal physical activity would cause spontaneous fractures and/or a bone pain syndrome, mainly affecting the spine; however, falls could also cause extremity bone fractures. (B) In a "physiologic osteopenia," reduced bone strength and "mass" would fit correspondingly reduced physical activities and muscle strength so well that fractures would not happen without falls or other injuries. Those fractures would affect extremity bones more than the spine. (C) In "combination states," features of (A) and (B) would combine variably. (D) "Transient osteopenias" would occur while serious injuries heal. After healing, transient osteopenias usually resolve without treatment, and fractures occur only from injuries. While an osteopenia's severity usually affects the risk of fracture, its pathogenesis could strongly affect the treatment needed for prevention or cure.