Arm and leg composition determined by anthropometry (AP, girths, and skinfolds) were compared with computed tomography (CT) in 7 young (mean age 31.4y) and 13 old (74.8y) males. Five transverse CT scans, girths, and skinfold measurements were made. In each CT image, total cross-sectional area (CSA), bone CSA, muscle compartment CSA, and "pure" muscle CSA within the compartment were measured. Corresponding volumes were also calculated. Total limb and muscle plus bone CSA and volumes were estimated from AP measures. The two methods were compared, and multiple regression analyses were used to predict component CSA and volumes from AP measures. In both groups, AP component measures in the leg provided more accurate values than in the arm. Skin plus subcutaneous tissue in the old was not well estimated by AP. The greater preponderance of nonmuscle tissue in the muscle compartments of the old, as determined from CT images, was not related to any AP estimates. Prediction equations for various component areas and volumes were derived. Some equations derived for the young could not be obtained for the old due to the reduced ability of AP to accurately assess limb composition of aged men.