Total body water (TBW) is an indicator of fat-free mass and thus of nutritional status but cannot be measured readily in children in developing countries who are at greatest risk of becoming malnourished. We therefore developed equations to predict 18O TBW from bioelectrical impedance (Z), wt, and ht in well and malnourished infants and children whom we considered characteristic of children evaluated in nutritional surveillances in Peru. Children 3 to 30 mo of age, whose wt were 3.4 to 14.4 kg, which was -2.8 to +1 SD wt-for-ht, were randomly assigned to group I (n = 30) to develop equations to predict TBW or to group II (n = 14) to cross-validate the predictive equations. Mean TBW measured by 18O dilution was 4.8 +/- 1.2 kg in group I, and 5.6 +/- 1.7 kg in group II. TBW ranged from 57 to 78% of body wt (65 +/- 6%) in group I and from 56 to 80% (64 +/- 6%) in group II, indicating no statistically significant differences in body composition. The following equation was developed and cross-validated: TBW, kg = 0.48 + 0.68 ht2/Z; standard error estimate = 0.36; r = 0.98). A slight improvement was achieved by the addition of body wt (TBW, kg = 0.76 + 0.18 ht2/Z + 0.39 wt; standard error estimate = 0.23; r = 0.99).