One hundred twenty-eight nonmalnourished male patients between 3 and 36 months of age were randomly assigned to receive one of four lactose-free dietary treatments to determine the effect of dietary therapy on the severity and nutritional outcome of diarrheal illness. Group 1 received a formula diet composed of casein, sucrose, dextrin with maltose (Dextri-Maltose), and vegetable oil to provide 110 kcal/kg body weight/d (CSO-110). Group 2 received CSO to provide 55 kcal/kg/d (CSO-55) for 2 days and then CSO-110. Group 3 received only oral glucose-electrolyte solution (GES) for 2 days, CSO-55 for the next 2 days, and then CSO-110. Group 4 received the same diets as Group 3 except that only intravenous GES was used for the first 2 days. The GES maintenance solutions provided 24 to 30 kcal/kg/d. Therapeutic success rates were similar among dietary groups, ranging from 90% to 97%. Fecal excretion was initially lower in group 4 (P less than 0.05) but was similar initially among groups treated orally and among all four groups beginning on day 3. Net apparent absorption of nitrogen, fat, carbohydrate, and total energy; retention of nitrogen; and increments in body weight, arm circumference, and skin-fold thickness were positively related to the amounts of dietary energy consumed. Thus continued oral feeding with the CSO diets during the early phase of therapy yielded improved nutritional results.