Soluble aspirin given by mouth in divided dosage decreased intestinal fluid loss in infants and young children with acute gastroenteritis. The treated group had significantly less diarrhea, which ceased earlier and needed less intravenous therapy, than a randomly selected control group given an indistinguishable placebo. This effect of aspirin occurred with diarrhea caused by Salmonella, Aeromonas, Escherichia coli producing heat-labile toxin, and rotavirus, but not with diarrhea associated with strains of E. coli producing heat-stable toxin.