The ability of anthropometric measurements, including arm muscle area calculated from arm circumference and triceps skinfold thickness, to estimate creatinine excretion as an index of muscle mass was tested in 14 fully active children, 2 to 6 yr of age in a residential care facility. The children were placed on a meat and fish-free diet for 36 to 48 h before the determination of mean creatinine excretion during three 24-h urinary collections. Results indicated highly significant correlations between mean creatinine excretion and height (r = 0.71), weight (r = 0.69), and arm circumference (r = 0.63). However, arm muscle area had the highest correlation with creatinine excretion (r = 0.86) of any anthropometric variable used. Step-wise multiple-regression analysis indicated that including height or weight with arm muscle area did not significantly improve the estimation of creatinine excretion, but that weight for height added significantly to improve the estimation of creatinine excretion, but that arm muscle area can provide a useful index of body muscle as reflected in urinary creatinine excretion in young children.