EVALUATION of age changes in the composition of the body is hampered by the inadequacy of current procedures for determining body composition. Although a number of techniques have been proposed, only a few of these have been validated by direct carcass analysis. Validation has been done only on experimental animals. In order to simplify the problem of determining body composition, the body may be said to be composed of the skeleton, fat, and fat-free tissue (muscle). For any individual, the weights of the skeleton and of fat-free tissue appear to be fixed within relatively narrow limits. The weight of body fat, however, covers a relatively wide range. The methods used in determining body composition provide, at best, estimates of the size of either the fat or fat-free tissue. Either one or both of these components may be measured, but usually only one is determined. The mathematical equations that have been developed for computing the percentage composition of the body contain factors that make allowances for the weight of the skeleton. Minerals are one of the chief components of the skeleton, but since minerals account for a relatively small percentage of the body weight, any variation in the size of this component will have only a minor influence on body composition. Although it would be desirable to know the changes that occur with age in all body components, most of the work in the field of body composition has been concentrated on determination of the amount of fat. This stems from the interest engendered during the past few years in obesity in general. Changes in the skeletal mass have been largely neglected. On the basis of clinical experience it has been proposed that changes do occur in the skeletal mass. Many clinicians working in the field of geriatrics are impressed by the fragility of the bones in their older patients. This has been assumed by some to be due to a negative calcium balance, with a consequent depletion of bone minerals. However, a recent study suggests that "the onset of bone fragility in the aged is a composite effect of many factors and is probably more intimately related to endocrine changes, decreased activity, and loss of muscle tone than to level of nutrition" (1).