Dual-energy X-ray absorptiometry (DXA) is a new method for assessing whole-body and regional bone, fat and fat-free mineral-free soft tissue masses. X-rays at two discrete energy levels are collimated and directed into the body. The attenuation of the X-rays by the various chemical components in the body permits determination of important compositional variables. The precision and accuracy of the DXA measurements of bone mineral content and density are 99% and < 1% error, respectively. The reproducibility of the determinations of soft tissue composition is approximately 99%. Because the radiation dose associated with the X-ray exposure is low, DXA is a safe method for routine use in humans. Various applications are described, including an assessment of bone mineral status and body composition in adults and infants. This method is appealing for nutritional studies because it directly determines the composition of bone and of the body.