Background: The use of hydrostatic weighing (HW) to measure body composition in the elderly can be difficult and is based on the assumption of constancy of body compartments. Objective: We calibrated and validated a new air-displacement plethysmography (AP) method for measuring body composition in the elderly. Design: A 4-compartment equation for calculating percentage body fat (%BF) that used body density (D b ), total body water, and bone mineral content was used as the criterion for evaluating %BF estimated by the 2-and 3-compartment models. . However, analysis of variance showed a significant two-way interaction between sex and compartment model (P < 0.02), indicating that the comparisons between the sexes were different across all compartment models. The %BF calculated for the women was significantly higher than that calculated for the men by both HW and AP and for all compartment models. Conclusion: Our data indicate that D b(AP) was not significantly different from D b(HW) . Although differences were seen in %BF between the sexes, we observed no significant differences among the compartment models within each sex for this group of older individuals.Am J Clin Nutr 2001;74:637-42.