Quantitative microradiography has been developed for studying bone turnover and has been applied to a study of normal and osteoporotic human bone. Evidence is presented that this method is reproducible and provides an accurate measure of bone formation and resorption. It was demonstrated that bone from the majority of osteoporotic patients differs from normal bone by an increase in the amount of resorption. Bone formation levels are generally normal; however, in Cushing's syndrome, after corticosteroid therapy, and after immobilization, bone formation decreases.