Follow-up of postmenopausal patients who have been on long-term estrogen replacement therapy has provided a large number of women for a prospective study of osteoporosis. The development of new and simple instrumentation has made it possible to evaluate bone changes easily and accurately in serial or survey studies. The degree of osteoporosis is determined by the transmission of low-energy radiation from radioactive iodine 125 through an in vivo bone sample. The measure used here is the linear absorption coefficient of overall bone of a phalanx. The results show that postmenopausal osteoporosis is prevented or retarded by estrogen replacement therapy as compared with an unsupplemented control group.