Dual-energy x-ray absorptiometry (DXA) is unquestionably the method of choice for the assessment of bone mass. Recent generations of DXA instruments also have the capability of assessing soft tissue mass, thereby providing a three-compartment model of body composition, i.e., bone mineral, fat, and bone-free lean masses. Although hydrodensitometry has long been considered the reference method of assessing body composition, assumptions regarding the constancy of the constituents of fat-free mass may compromise its validity in some populations. Because DXA appears to be less dependent on assumptions regarding biological consistency, it is possible that it has the potential to provide a more accurate assessment of body composition across populations than does hydrodensitometry, and that it should be considered the reference method. DXA has been shown to provide precise measures of body composition, but it remains questionable as to whether those measures are accurate. Contributing to the uncertainty regarding validity is the variability among manufacturers of DXA instruments in the methods of calibration, data acquisition, and data analysis. Although DXA holds great promise in becoming the criterion method of assessing body composition, and has been promoted as such by some investigators, available data indicate that this endorsement is premature.