ExtractLean body mass (LBM) was estimated by40K counting in 609 normal boys and girls 7.5–20.5 years of age. This component of the body is shown to be related to stature, but the quantitative nature of the relation varies with age and sex.Boys have a higher LBM/height ratio in adolescence than do girls, and the slope of the LBM-height regression is also greater, whereas before adolescence there is no appreciable sex difference. Age also affects LBM independently of stature. This effect is first seen during adolescence, and it is more pronounced in boys.When the entire age span for this group of subjects is considered, the relation between LBM and height is exponential, LBM = b.ek.ht, b and k being constants. This means that the relative, or percentage, growth in LBM is a linear function of height growth at this time of life.SpeculationThe relations developed from these data emphasize the need to gear nutritional requirements to the speed of height growth. On the average, the adolescent boy has greater needs than does the girl, merely to satisfy the demands of the growth process. The changing pattern of the LBM-height relation during adolescence lends support to the concept that androgens facilitate the growth of the LBM with its large component of muscle.