OSTEOPOROSIS is an enormous public health problem, responsible for at least 1.2 million fractures in the United States each year. The sites of these fractures are the vertebrae in 538,000 cases, the hip in 227,000, the distal forearm (Colles' fracture) in 172,000, and other limb sites in 283,0001 (and Melton LJ, Riggs BL: unpublished data). One third of women over 65 will have vertebral fractures (Melton LJ, Riggs BL: unpublished data). By extreme old age, one of every three women and one of every six men will have had a hip fracture.² This catastrophic type of fracture is fatal in . . .