There is no doubt that body composition changes with aging. Some general trends have been described, including an increase in body weight and fat mass in middle age followed by a decrease in stature, weight, FFM, and body cell mass at older ages. Losses in muscle, protein, and bone mineral contribute to the decline in FFM; however, the onset and rates of decline remain controversial. Most data are available for men and women < 80 yr and we know relatively little about the normal status and the changes that occur in body composition in elderly men and women. This situation has developed in part because the changes that occur in various body constituents with aging confound the estimation of body composition by traditional techniques. Hence, there is a need for longitudinal reference data in persons 80 yr of age, both to describe the normal status and to develop valid prediction equations for estimating body composition in older men and women in settings outside the laboratory. This should be possible using new technologies and approaches based on multiple component models of body composition. An understanding of the normal changes in body composition with increasing age, the normal variation in these changes, and their health implications is important for the health, nutritional support, and pharmacologic treatment of elderly men and women in the United States. The information is especially important because elderly men and women, in terms of both numbers and health care dollars, represent the most rapidly expanding segment of the U.S. population.