Refeeding of infants with acute diarrheal disease. The purpose of this study was to determine which infant formula among five would be the most efficacious for the refeeding of infants during an acute episode of diarrhea. Fifty male infants less than 12 months of age with severe diarrhea and at least 5% dehydration were admitted to a metabolic unit and studied in a prospective, single-blind protocol. Ten infants randomly received one of five types of formula: two-thirds diluted cow milk, cow milk formula (Nanon, Nestle, Inc., Sao Paulo, Brazil), Portagen, Pregestimil, or Prosobee (Mead Johnson & Co. Division, Evansville, Ind.). They continued to receive the same formula for 72 hours unless dehydration occurred. There were no associated infections, and they received no prior antibiotic treatment. Oral hydration together with intravenous fluid therapy was given to all patients during the initial treatment. During the first 72 hours of refeeding, patients fed Portagen excreted the least amount of stool and required reduced quantities of intravenous fluids or oral hydration. In contrast, patients fed diluted cow milk or any other formula had more severe diarrhea. Nine of the 10 patients fed Portagen completed the 72-hour treatment, whereas only 2 of 10 fed diluted cow milk tolerated it. Similarly, the cumulative proportions for high purging rate, dehydration, carbohydrate intolerance, and vomiting were more favorable for Portagen and least acceptable for diluted cow milk. No differences were found among the remaining three formulas tested. These data show that diluted cow milk is poorly tolerated by infants with severe diarrhea, whereas Portagen is more effective. 