A hypotonic (osmolality 224 mmol/l, sodium 60 mmol/l) oral rehydration solution (ORS) was compared with an isotonic high glucose ORS (osmolality 304 mmol/l, sodium 60 mmol/l) in children with acute diarrhoea in a randomised double blind study. The stool output and hence the mean consumption of ORS for maintenance hydration was less (p = 0.036) in patients receiving hypotonic (69 ml/kg) than isotonic (97 ml/kg) ORS. Hypotonic ORS was more effective in patients with rotavirus positive than with rotavirus negative diarrhoea.