New specific markers of bone remodelling have been developed that allow the evaluation of bone formation (plasma levels of osteocalcin and bone alkaline phosphatases) or bone resorption (collagen crosslink levels in urine). These markers can be used to evaluate bone disease. Their best application is currently in renal osteodystrophy, where there is a broad spectrum of bone abnormalities. In patients with this disease, bone markers can obviate the need for bone biopsy. So far, bone alkaline phosphatase is the most sensitive and specific marker for predicting the type of bone disease and therefore for deciding treatment in dialysed patients, although it is not ideal. Bone markers have also allowed a dramatic improvement in the comprehensive approach to bone loss with aging in women. Monitoring of the marked increase in bone turnover, and its persistence, in older women after menopause may help in the design of treatment strategies for osteoporosis. There is some hope that the measurement of urinary levels of collagen crosslinks will help to predict the clinical outcome of osteoporosis with respect to fractures in postmenopausal women. Bone markers could, besides being a measurement of bone density, help to determine the optimal treatment of postmenopausal women. Bone markers could also be used to predict or ascertain the response to treatment of osteoporosis, but few data are currently available to judge the routine usefulness of these new applications.