In 1994, the World Health Organization published diagnostic criteria for osteoporosis. Since then, many new technologies have been developed for the measurement of bone mineral at multiple skeletal sites. The information provided by each assessment will describe the clinical characteristics, fracture risk and epidemiology of osteoporosis differently. Against this background, there is a need for a reference standard for describing osteoporosis. In the absence of a true gold standard, this paper proposes that the reference standard should be based on bone mineral density (BMD) measurement made at the femoral neck with dual-energy X-ray absorptiometry (DXA). This site has been the most extensively validated, and provides a gradient of fracture risk as high as or higher than that of many other techniques. The recommended reference range is the NHANES III reference database for femoral neck measurements in women aged 20-29 years. A similar cut-off value for femoral neck BMD that is used to define osteoporosis in women can be used for the diagnosis of osteoporosis in men - namely, a value for BMD 2.5 SD or more below the average for young adult women. The adoption of DXA as a reference standard provides a platform on which the performance characteristics of less well established and new methodologies can be compared.