Sum of segmental bioimpedance analysis (BIAs) has recently been introduced as a more accurate estimation of extracellular volume (ECV). The current study was designed to compare the changes in ECV estimated by whole body bioimpedance (BIA(W)) and BIAs and to determine whether BIAs could be used to accurately estimate changes in ECV compared with the ultrafiltration volume (UFV) as measured by the dialysis machine. Ten men (age, 49 +/- 10 years; dry weight, 78 +/- 13 kg) were studied during hemodialysis (HD). A multifrequency bioimpedance analyzer (BIS4000B; Xitron Technologies, San Diego, CA) and a digital switch developed by the authors were used to automatically collect data from three body segments (arm, trunk, and leg) and the whole body using the stated technique. There was a significant difference in delta ECV(W) and UFV at the end of the treatment (2.75 +/- 0.74 L vs 3.95 +/- 0.73 L, p < 0.05) because fluid removal from the trunk has almost no effect on whole body resistance. However, delta ECVs was not significantly different from UFV (3.76 +/- 0.65 L vs 3.95 +/- 0.73 L, p = NS). BIAs is a more accurate approach to monitor changes in ECV during HD than is BIA(W) because changes in local resistance can be allocated to segments with uniform geometry and resistivity. This is a prerequisite for future studies on dynamics of regional fluid distribution during hemodialysis.