Data from the 1990 survey of the National Study of Health and Growth, comprising 3357 white English boys and 3050 white English girls, were used to construct and evaluate a new index of weight-for-height. Prediction of triceps + subscapular skinfold thickness, using half of the data, led to the index (weight -9)/height3.7, where weight was in kilograms, height in metres. Using the second half of the data the index was shown to have almost as good a correlation with normalized (triceps + subscapular skinfold) as the body mass index (BMI) in children aged 4-12 years. The new weight-for-height index had stable variance from age 4 to 12 years after simple log transformation, unlike the BMI which required transformation via three age-related parameters prior to analysis or centile calculation. Neither index was a good proxy for skinfold measurements for comparison of ethnic groups. From data from inner city areas surveyed in 1989 children of Indian subcontinent origin had substantially lower weight-for-height, but only Gujarati children were thinner than white children, whereas Afro-Caribbean children were slightly heavier for their height, but thinner. Centiles of weight-for-height, and of BMI by age, for ages 4-12 years, were calculated for white English boys and white English girls.