A hypotonic oral rehydration salts (ORS) solution with total osmolality of 224 mosmol/l was compared in an open clinical trial with an isotonic (osmolality 304 mosmol/l) ORS solution for the treatment of dehydration due to acute diarrhoea. Both ORS solutions had the same electrolyte composition with a Na+ concentration of 60 mmol/l. Children given the hypotonic ORS solution (n = 103) passed significantly fewer diarrhoeal stools, and their diarrhoea and hospital stay were shorter than those of children given the isotonic ORS solution (n = 135). We conclude that hypotonic ORS ("light" ORS) has clinical advantages over the standard ORS currently used in Finland.