Our concepts of the pathogenesis, diagnosis, prevention, and treatment of osteoporosis are radically changing. Some changes, such as the study of genetic determinants of bone mass and turnover and the identification of local factors in pathogenesis, have just begun. The use of bone densitometry to diagnose and predict fracture risk is well developed but not yet widely applied. Measurement of bone turnover done by using biochemical markers is a promising new diagnostic method that has already proved useful in assessing a patient's response to therapy. Options for prevention and treatment have increased substantially with the Food and Drug Administration's recent approval of alendronate (a bisphosphonate) and nasal calcitonin for treatment of osteoporosis. Some are concerned that these new agents will unduly reduce the use of estrogen, which should remain the mainstay for prevention of bone loss and fractures in postmenopausal women. New therapeutic approaches are needed to treat the established disease. Our goal should be to develop inexpensive and widely applicable methods for diagnosis, prevention, and treatment to limit the enormous increase in osteoporotic fractures that has been predicted as the aging population expands worldwide.