There is concern that feeding full-strength animal milk to infants aged less than 6 months with diarrhoea may have adverse consequences. We assessed the effects on clinical course of two feeding regimens in 159 Guatemalan and Brazilian infants aged 2 weeks to 6 months who had had acute diarrhoea for 120 h or less, showed signs of mild to moderate dehydration, and had no complications. After correction of dehydration, infants were assigned randomly to receive continued full-strength milk feeding or initial feeding with diluted milk with regrading to full-strength milk over 48 h. There were no significant differences between feeding groups in rate of treatment failures (-1%, 95% Cl -14 to 12%) or mean (SD) total stool output (full-strength milk 335  g/kg, diluted milk 338  g/kg) and duration of diarrhoea (92  vs 92  h). A significant association was found between presence of reducing substances in stools and treatment failure (OR 4.3, 95% CI 1.1 to 16.8), but reducing substances in stools were common both in treatment successes (61%) and in failures (87%). Our study supports the conclusion that, for infants under 6 months of age with diarrhoea whose only food is animal milk or formula, the milk or formula normally given should be provided in full strength as soon as dehydration has been corrected.