Total body water can be calculated using stable isotopes by two alternative approaches known as the plateau method and the back extrapolation method. These methods treat the isotopic data in different ways and consequently do not necessarily produce identical values for total body water. Total body water was therefore calculated using both methods using two different tracers, 2H and 18O.A cohort of infants was studied at three ages in the first year of life, and the two methods and two tracers compared at each age.The plateau method was found to produce values for total body water greater than those obtained by the back extrapolation method at all three ages using both isotopes (P < 0.01 in all cases).This difference is due to the different time points at which total body water is calculated. Back extrapolation values correspond more closely to values of total body water obtained by alternative techniques.