Multifrequency bioelectrical impedance was used to predict changes in body water compartments during renal dialysis. Weight loss during dialysis was assumed to be water loss. Predicted total body water (TBW) from impedance after dialysis did not differ significantly from TBW determined by deuterium oxide dilution. However, the predicted change in TBW from bioelectrical impedance largely exceeded the observed weight (water) loss. The predicted change in extracellular water was slightly but significantly lower compared to the observed weight loss. The ratio of impedance at 1-100 kHz increased in all subjects during dialysis, and may be a simple tool to assess body water distribution.