Accurate estimation of total body water (TBW) is a critical component of dialysis prescription in peritoneal dialysis (PD). Gold-standard isotope dilution techniques are laborious and costly; therefore, anthropometric prediction equations that are based on height and weight are commonly used to estimate TBW. Equations have been established in healthy populations, but their validity is unclear in children who undergo PD, in whom altered states of hydration and other confounding alterations in normal physiology, particularly retarded growth and pubertal delay, may exist. TBW was measured by heavy water (H2O18 or D2O) dilution in 64 pediatric patients who were aged 1 mo to 23 yr and receiving chronic PD in the United States and Germany to establish and validate population-specific anthropometric TBW prediction equations and to compare the predictive power of these equations with formulas that have been established in healthy children. The best-fitting equations are as follows: For boys, TBW = 0.10 × (HtWt)0.68 − 0.37 × weight; for girls, TBW = 0.14 × (HtWt)0.64 − 0.35 × weight. The height × weight parameter also predicts body surface area (BSA). These equations can be simplified, with slightly less precision, to the following: For boys, TBW = 20.88 × BSA − 4.29; for girls, TBW = 16.92 × BSA − 1.81. TBW is predicted without systematic deviations and equally well in boys and girls, North American and European, obese and nonobese, growth-retarded and normally sized, and pre- and postpubertal children. In contrast, previous anthropometric equations that were derived from healthy children systematically overpredicted TBW and were less precise in this pediatric PD population. In summary, a new set of anthropometric TBW prediction equations that are suited specifically for use in pediatric PD patients have been provided.