Height and weight data from children in the United States, United Kingdom, Japan, and Singapore were analyzed to investigate differences in growth between the groups of children. An investigation into the adjusted weight indexes of the form index = weight/heightp for differing powers of p (Benn index) showed that the power of p required to produce a correlation of zero between the index and height varied with age. For the United States, Japan, and Singapore the p value was just below 3.0 for children aged 6 y, increased to approximately 3.5 for children aged approximately 10 y, and decreased to approximately 2.0 at age approximately 18 y. A consequence of the p value being mostly >2.0 is that BMI (wt/ht2) tends to be greater for tall children than for short children. The US data (from the second National Health and Nutrition Examination Survey) also contained information on skinfold thickness. Relating skinfold thickness to indexes of the same form for height and weight suggested that the best relation was achieved with p values of approximately 2.0 except for children aged 12-16 y, for whom the optimal values for p were higher. The highest value, 2.9, was achieved at ages 12-13 y. Overall, the use of BMI as an indicator of adiposity appears acceptable for children aged 6-7 and 17-18 y. However, BMI should be used with caution when assessing children aged 8-16 y.