An epidemic of gastroenteritis during which an unusually high incidence of hypertonic dehydration was encountered in infants is reported. Analysis of the feedings received by the infants during the initial phase of diarrhea indicates that those who developed hypernatremia had received feedings in which the ratio of solutes to water was higher than that in the estimated continuing losses of fluid through stools, urine, and skin and lungs. Considerations to be employed in the prevention and treatment of hypernatremia and dehydration during diarrhea are discussed. The physician must develop his clinical judgment in determining when parenteral rather than oral therapy is required in diarrhea—and not depend on a routine regimen or solution for oral therapy.