Data from three American Health Examination Surveys and 11 other published studies were used to investigate the weight-for-height index W/Hp, adjusted for age and sex by expressing it as a fraction of the same ratio for an NCHS standard child. The appropriate power of height p was determined by regressing log weight-for-age on log height-for-age, giving an index of relative weight which is highly correlated with weight but uncorrelated with height for age. The optimal value of p was 2 in pre-school children, but it increased gradually to 3 at age 11 and fell back to 2 after puberty. The largest value of p occurred 18 months earlier in girls than in boys, and the pattern was the same for white and black children. The trend was summarized by the formula: p = 2 + exp [-0.5 (age - 11)2]. The index thus provides a compact way of adjusting weight for height and age throughout childhood, using standards of weight and height for age and sex. During puberty, W/Hp was found to be related to maturity as measured by Greulich-Pyle bone age. Each year's advancement relative to chronological age was associated with an increase of up to 5% in relative weight, adjusted for height and age. This maturity effect was shown to be the cause of the rise in the value of p early in puberty, so that after adjusting for it the value of p remained close to 2 throughout childhood. Thus the index W/H2 is appropriate for preschool children and adults, but early in puberty it tends to assess tall or physically advanced children as being overweight. This bias can be avoided by using instead the more general index, where the precise power of height is a function of the child's age.