Measurement of bioimpedance (BI) is a simple non-invasive technique that relies on the different conductivity of tissues to define body composition and can be easily adapted to automated monitoring. We assessed the accuracy of Bl in monitoring rehydration and acute fluid fluxes in 35 Peruvian cholera patients. Patients were monitored throughout the acute phase of diarrhoea and followed up at 3 and 10 days. Bl was compared with other objective measures of dehydration including packed cell volume, serum protein, and calculated fluid balance. Bl rapidly detected inadequate treatment and acute fluid flux, correlating highly with intravascular hydration as measured by serum protein and packed cell volume. Bl values during dehydration were significantly raised compared with 10-day convalescent values and age-matched controls (p<0·05). We also encountered an unexpected difference in the bioelectrical response to dehydration and rehydration between sexes. We conclude that Bl has uses in monitoring dehydrated patients, in oral rehydration trials, and in physiological studies. © 1993.