This study evaluated whether aging alters the usefulness of creatinine excretion as an index of lean body mass (LBM) or muscle mass in healthy men and women. Creatinine excretion was determined while 24 young (15 men and 9 women aged < 32 y) and 38 older (23 men and 15 women aged > 60 y) healthy volunteers stayed at a clinical research center for 3 d. Total LBM was determined by 40K counting in all subjects, and cross-sectional areas of upper arm and thigh muscles were determined in some subjects (10 young, 20 old) by magnetic resonance imaging. The slopes and intercepts of the regression equations relating LBM to average daily creatinine excretion were the same in the younger and older groups, and the precision with which LBM could be determined from creatinine excretion was not diminished by aging. Creatinine excretion was closely correlated with cross-sectional areas of upper arm (r = 0.85) and thigh (r = 0.88) muscles, and the slopes and intercepts of the regression equations were not significantly affected by age. The older group had a mean LBM smaller than that of the younger group, which was accounted for entirely by a smaller muscle mass. Mean resting metabolic rate (RMR), whole-body protein turnover rate, and thigh strength were lower in the older group than in the young group. Adjustment of RMR and protein turnover for creatinine excretion or LBM eliminated the age effects. Strength was lower in the older group even after adjustment for creatinine excretion or LBM. This study indicates that creatinine excretion is useful for evaluating body composition in both young and old subjects.