Background: In paediatric clinical practice treatment is often adjusted in rel ation to body size, for example the calculation of pharmacological and dialysis dosages. In addition to use of body weight, for some purposes total body water ( TBW) and surface area are estimated from anthropometry using equations developed several decades previously. Whether such equations remain valid in contemporary populations is not known. Methods: Total body water was measured using deuteriu m dilution in 672 subjects (265 infants aged < 1 year; 407 children and adolesce nts aged 1-19 years) during the period 1990-2003. TBW was predicted (a) using published equations, and (b) directly from data on age, sex, weight, and height. Results: Previously published equations, based on data obtained before 1970, si gnificantly overestimated TBW, with average biases ranging from 4%to 11%. For all equations, the overestimation of TBW was greatest in infancy. New equations were generated. The best equation, incorporating log weight, log height, age, an d sex, had a standard error of the estimate of 7.8%. Conclusions: Secular trend s in the nutritional status of infants and children are altering the relation be tween age or weight and TBW. Equations developed in previous decades significant ly overestimate TBW in all age groups, especially infancy; however, the relation between TBW and weight may continue to change. This scenario is predicted to ap ply more generally to many aspects of paediatric clinical practice in which dosa ges are calculated on the basis of anthropometric data collected in previous dec ades. Background: In pediatric clinical practice treatment is often adjusted in rel ation to body size, for example the calculation of pharmacological and dialysis dosages. In addition to use of body weight, for some purposes total body water (TBW) and surface area are estimated from Methods: Total body water was measured using deuteriu m dilution in 672 subjects (265 infants aged <1 year; 407 children and adolescents aged 1- Results: Previously published equations, based on data obtained before 1970, Table of Contents Previously published equations, and (b) directly from data on age, sex, weight, and height. si gnificantly overestimated TBW with average biases ranging from 4% to 11%. For all equations, the overestimation of TBW was greatest in infancy. New equations were generated. The best equation, incorp orating log weight, log height, age, an d sex, had a standard error of the estimate of 7.8%. Conclusions: Secular trend s in the nutritional status of infants and children are altering the relation be tween age or weight and TBW. Equations developed in previous past significant ly overestimate TBW in all age groups, especially infancy; however, the relation between TBW and weight may continue to change. This scenario is predicted to ap ply more generally to many aspects of paediatric clinical practice in which dosa ges are calculated on the basis of anthropometric data collected in previous dec ades.