X-ray-based procedures are available to measure bone mineral density in vitro at almost any skeletal site. These bone density measurements are not useful in the diagnosis of the cause of bone loss but at present are the only tests available for assessing bone mass prior to the occurrence of irreversible changes such as fractures or vertebral compression, which are easily recognizable on x-rays. When fractures are present, the severity of the bone loss and the risk for future fractures can be assessed. Repeated measurements permit estimation of the rate of bone loss, which gives useful information for monitoring treatment effect or course of the disease. Measurement of total body calcium is of less clinical importance because of the predominantly trabecular bone loss that generally occurs in metabolic bone disease. Dual-energy x-ray absorptiometry (DEXA) and quantitative computed tomography (QCT) of the spine are of about equal clinical value in the first approach to the patient with metabolic bone disease, although DEXA allows greater variety in sampling sites. For repeated measurements, DEXA provides better precision at significantly lower radiation burden. For bone mineral measurements, the lumbar spine appears to be the most sensitive skeletal site.