Total body impedance (TBI) is used to monitor alterations in total body water (TBW), e.g. during haemodialysis. However, 80% of the TBI signal is due to the resistance in extremities. Therefore TBI measurements give little information about alterations in thoracic fluid content (TF). Measuring both total body impedance and thoracic impedance (THI) can be a useful method to monitor TBW and TF changes during haemodialysis. In this study TBI and THI measurements were performed during 30 dialysis sessions and also in one group of 24 control subjects with normal tissue hydration. During haemodialysis TBI and THI significantly increased, demonstrating a decrease of TBW and TF. The ratio THI/TBI did not differ significantly during haemodialysis, indicating a similar effect of ultrafiltration on TBW and TF. The finding that TF decreased during haemodialysis was rather surprising, whereas pulmonary oedema is not an evident clinical problem in patients on maintenance haemodialysis treatment. THI measurements after dialysis proved to be significantly higher in comparison to the control subjects. This finding supports the hypothesis that the decrease in TF during dialysis is partly due to the sudden ultrafiltration-induced hypovolaemia leading to a fall in thoracic blood volume. BACKGROUND UF volume, ultrafiltration volume; TBI, total body impedance; THI, thorax impedance; TBW, total body water; TF, thoracic fluid.