Bone densitometry is now used widely in studies of bone mass. In just a few years, as a result of technological developments and efforts to control health care costs by using densitometric methods in the prevention of bone disease, mainly osteoporosis, we have progressed from simple forearm densitometry to total body densitometry with vertebral morphometry. However , recent studies show that radiogrammetry, a simple and inexpensive radiologic technique that is available at any hospital, is as effective as spinal densitometry, the technique most widely used in bone mass studies [1, 2]. Regardless of the value of bone densitometry in bone mass studies, its results can be expressed as either total body bone mineral content (TBBMC) and regional bone mineral content (RBMC), both in grams, or the more frequent total body bone mineral density (TBBMD) and regional bone mineral density (RBMD), both in g/cm 2. A recent development is the measurement of volumetric bone mass by axial or peripheral quantitative computed tomography (in g/cm 3) [3]. Regardless of how the measurements are expressed, normalization of bone mass to g/cm 2 or g/cm 3 minimizes sexual differences. Mazess et al. [4], prolific researchers of densitometric measurement of bone mass, find that sexual differences that are evident when bone mass measurements are expressed as grams of regional BMC are unapparent when bone mass is normalized for density. Although we have no wish to enter into debates, we agree with Dequeker and Geusens [5] that bone mass measurements should be expressed as BMC rather than BMD because BMC better discriminates the sexual differences in bone mass. Moreover, if skeletal calcium content is what really matters, TBBMC informs us of the absolute amount of total calcium (TBCa) because skeletal calcium represents a constant proportion of total mineral content [6]. As such, TBBMC is undoubtedly important in some disorders. We find that another advantage of expressing bone mass as regional BMC is that it correlates better with bone resistance to fracture than does regional BMD [7]. Changes in bone mass in men and women are influenced significantly by changes in weight [8, 9], weight being the principal determinant of bone mass in women [10]. Because of possible fluctuations in weight in the course of longitudinal studies [ l l ] , bone mass measurements should be evaluated in the light of changes in other body components. This has not been done in most studies, which overlook the possible relation between variations in bone mass and changes in weight. In fact, even over short time intervals, changes in