There is a conflict in previous studies with regard to the relation between adipose tissue mass and total body fluid distribution. This study tested the hypothesis that obesity is accompanied by an increase in the extracellular-to-intracellular fluid ratio above that observed in nonobese subjects. Extracellular fluid was evaluated in obese (n = 39) and nonobese (n = 26) healthy women, using two different dilution volumes, 35SO4 [extracellular water (ECW)] and 24NaCl [exchangeable sodium (Nae)]. Intracellular water (ICW = 3H2O dilution volume-ECW) and total body potassium (TBK; 40K whole body counting) were assumed to represent intracellular fluid. Two independent markers of relative fluid distribution were formulated as ECW/ICW and Nae/TBK. Obese and nonobese women were of similar age and height but differed in body weight and TBW by 67.7 kg and 12.9 liters, respectively. The obese women had significantly larger absolute ECW, Nae, ICW, and TBK compared with the nonobese women (all P less than 0.001). The ratios ECW/ICW and Nae/TBK were significantly higher in obese vs. nonobese women and were highly correlated with each other (r = 0.54, P less than 0.001) in the pooled group of subjects. Fluid volumes are thus increased in obese women, and the expansion is relatively greater for the extracellular compartment. These results have implications in the study of human body composition and may also account in part for the fluid-overload states that often accompany severe obesity.