The purpose of this study was to compare multifrequency bioimpedance spectroscopy (BIS) estimates of extracellular water volume (ECW) in critically ill patients with measurements by bromide dilution. Stable bromide dilution and BIS were performed in 37 critically ill patients as soon as haemodynamic stability was achieved (day 0) and again 10 days later. While BIS underestimated the dilution results on each day of measurement, the 10-day changes in ECW agreed closely for the two methods (4.42 +/- 4.25 (s.d.) vs 4.43 +/- 4.84 1).