ABSTRACT. A 4-month-old male infant with severe hyponatremic dehydration due to an enteropathogenic E. coli O125:B15-induced diarrhoea had continued very high stool fluid output with a very elevated sodium concentration after hospitalization and the institution of oral rehydration therapy (ORT). Thirty-six hours after start of ORT intravenous therapy was required. The results of studies of fluid and salt homeostasis in this patient have been compared with those obtained in 3 other patients who had acute diarrhoea of the same severity but caused by another strain of enteropathogenic E. coli (0111:B14) and who were successfully treated with ORT. On ORT the patient with treatment failure had a stool volume which was almost 8 times larger and a stool sodium output which was about 5 times higher than in the successfully treated patients. During the 36-hour-period of ORT fluid losses were about the same as the fluid intake. The results as regards urinary fractional sodium excretion and the urinary potassium/sodium quotient indicate that the severe sodium depletion which was present on admission in the unsuccessfully treated patient persisted during ORT. The reason for ORT failure may be that the infectious E. coli strain had bacilli-adherent qualities that cause damage of microvilli.