The composition of milks usually fed to 25 infants admitted to hospital with a dehydrating illness was studied. 15 hypernatraemic babies had been given feeds of greater sodium concentration and osmolality than those fed to the 10 infants whose plasma sodium was below 150 mEq/l. Hypernatraemic dehydration may be followed by death or permanent brain damage. Most infants in the survey were receiving milk with a sodium content greater than that advised by the manufacturers. Suggestions are made for reducing the sources of error commonly made in the reconstitution of dried milk formulae.