Almost all formulae for the prediction of total body water (TBW) from body impedance are based on the assumption of a constant conductor configuration--i.e. a constant subject section. In this paper we report on data obtained for a group of 19 young obese subjects (relative weight greater than 120%) and 10 young normal subjects (relative weight 80-110%). In obese subjects, the application of two different formulae generated from normal children gave biased results and led to an underestimation of TBW with respect to the reference value obtained by deuterium oxide dilution. Body mass index accounted for more than 40% of the inter-individual variability, suggesting that body size was not taken sufficiently into consideration by the predictive formulae used. We have used the body surface area as the anthropometrical parameter for the prediction of TBW from body impedance. The regression formula that we propose (TBW = 1.156 x (surface area/body impedance) - 2.356; R = 0.96), although requiring further validation on external populations, seems to provide a more realistic assessment of TBW in young obese subjects. We therefore suggest that the assessment of TBW in young obese subjects requires specifically designed prediction formulae.