Aging is associated with the onset of chronic diseases that lead to pathological expansion of the extracellular water (ECW) compartment. Healthy aging, in the absence of disease, is also reportedly accompanied by a relative expansion of the ECW compartment, although the studies upon which this observation is based are few in number, applied different ECW measurement methods, included small ethnicallyhomogeneous subject samples, and failed to adjust ECW for non-age related influencing factors. The aim of the current study was to examine, in a large (n=1538) ethnically diverse [African-American (AA), Asian, Caucasian, Hispanic] subject group the crosssectional relationships between ECW and age after controlling first for other potential factors that may influence fluid distribution. ECW and intracellular water (ICW) were derived from measured total body water (TBW; isotope dilution) and potassium (TBK; ⁴⁰K whole-body counting). The cross-sectional relationships between ECW, intracellular water (ICW), and ECW/ICW (E/I) and age were developed using multiple regression modelling methods. Body weight, weight², height, age, sex, race, and interactions were all significant ECW predictors. The slope of the observed race x age interaction was significantly greater in AA ( = 0.0005, p = 0.005) than in the three other race groups. Race, sex, and age differences in fluid distribution persisted after adjusting for body composition in a subgroup (n=994) with dual-energy x-ray absorptiometry lean soft tissue and fat measurements. A relative ECW expansion (i.e., E/I) was present with greater age in most sex-race groups, though the effect was not significantly larger in AA males (p>0.05) compared to the other race groups, except Asians (p<0.05). For females, a larger E/I -age effect was found in AA compared to the other race groups but only the comparison against Hispanics was significant (p<0.05). The ECW compartment and E/I are thus variably larger, according to race, in healthy older subjects independent of sex, lean soft tissue, and fat mass.