Mathematical equations using tetrapolar bioelectrical resistive (R) and reactive (Xc) impedance measures were developed and crossvalidated to predict total body water (TBW) and corrected bromide space (CBS) in two independent samples (n = 110). Height2 per low R was the best predictor of TBW (R = 0.96) and CBS (R = 0.92). When the influence of TBW was removed from CBS and dependent variables, height2 per low Xc was the best predictor (R = 0.50) of CBS. Double crossvalidation of each sample showed that observed and predicted TBW (R = 0.978 and 9.986) and CBS (0.937 and 0.907) were significantly related (p less than 0.001), and there was no difference (p greater than 0.05) between the values. The lines representing the relationships between observed and predicted values had regression coefficients not different than the line of identity. Data from both samples were combined to give a representative multiple regression equation to predict TBW and CBS. This study establishes the validity of the tetrapolar bioelectrical impedance method to assess body fluid volumes in humans.