For healthy children up to 15 years of age, no data exist regarding water turnover as directly measured using stable isotope techniques. Water turnover was measured in 171 healthy children (88 girls, 83 boys, age: 6 weeks-15 years) whilst living in their normal environment. Water turnover was calculated from the equilibrium of 2H2O concentration reached in the urine 2-3 h after an oral test load of 2 ml 2H2O/kg and the subsequent elimination rate of 2H2O. Daily water turnover decreased from 160 ml/kg (3 months), 100 ml/kg (12 months), 65 ml/kg (3 years) to 40 ml/kg at 15 years of age. Fluid intake was then calculated by subtracting the estimated metabolic water production from water turnover and this data was compared with recommended values. Our calculation of spontaneous daily fluid intake for these healthy children is within the recommended range during the 1st year of life, whereas for older children the recommendations exceed the spontaneous intake by a factor of 1.2-2.