Children with severe malnutrition have an increased risk of death from diarrhoea. To determine if the clinical manifestations of specific types of diarrhoea differed according to the nutritional status or size (weight and length) of the patient, we studied children with acute diarrhoea associated with rotavirus or enterotoxigenic Escherichia coli. In this study we found that a child's body size, which was determined by his age and nutritional status, was a significant predictor of his rate of stool output per kilogram of body weight. Thus, children who are small because of young age and/or malnutrition appear to lose a greater proportion of their total fluid volume during diarrhoea and might be expected to have a higher frequency of severe dehydration and death, if untreated.