Children with mild acute gastroenteritis have not been observed for specific evidence of lactose intolerance yet are frequently fed a nonlactose formula. To determine whether such intervention is justified, 85 infants with mild acute gastroenteritis were followed prospectively. Infants were blindly and randomly assigned to 20 calorie/oz formula containing one of four carbohydrates: lactose, sucrose, polycose, or combined sucrose-polycose. Daily diaries were kept by parents, and patients were reexamined on days 2, 7, and 14 of the study. Evidence for rotavirus was detected in 23 infants, and five had bacterial pathogens. Symptoms resolved in most patients within 7 days, but five infants were subsequently hospitalized. Stool frequency, weight gain, and need for hospitalization did not differ significantly among the groups. Recovery from mild acute gastroenteritis occurred within 2 weeks irrespective of carbohydrate ingested.