The traditional method of assessing total body water (TBW), extracellular water (ECW), and intracellular water (ICW) has been the use of isotopes, on the basis of the dilution principle. Although the development of bioelectrical impedance techniques has eliminated many of the measurement constraints associated with the dilution methods, the degree of interchangeability between the two methods remains uncertain. We used multifrequency bioelectrical impedance spectroscopy (BIS), 2H2O dilution, and bromine dilution to assess TBW, ECW, and ICW in 469 healthy subjects (248 males, 221 females) aged 3-29 yr. We found that the TBW, ECW, and ICW estimates for the BIS and dilution methods were significantly correlated (r2 = 0.80-0.96, P < 0.0001, SE of the estimate = 2.3-2.7 liters). On the basis of population, the constants used in the BIS analysis could be adjusted so that the mean differences with the dilution methods would become zero. The SD values for the mean differences between the dilution and BIS methods, however, remained significant for both males and females: TBW (+/-2.1-2.8 liters), ECW (+/-1.4-1.6 liters), and ICW (2.0-3.1 liters). To improve the accuracy of the BIS measurement for an individual within the age range we have examined, further refinement of the constants used in the BIS analysis is needed.