Different combinations of fluid therapy, feeding regimen, and cholestyramine were compared in search for optimal treatment of infants hospitalized for acute diarrhea. The infants (n = 81) received either rapid oral rehydration using the oral rehydration solution-World Health Organization formula (sodium 90 mmol/L, ORS-WHO) or traditional oral fluid replacement using a commercial glucose-electrolyte solution (sodium 35 mmol/L). One-half of the infants in both groups received full feedings at 24 h of hospitalization; in the remaining infants, feedings were gradually introduced over a period of 5 days. In addition, all the children were randomized to receive either cholestyramine 2 g four times daily or an equivalent amount of placebo. Rehydration with ORS-WHO, but not traditional fluid replacement therapy, led to correction of initial metabolic acidosis after 6-10 h; no cases of hypernatremia were observed with the use of ORS-WHO. Rapid return to full feedings appropriate for age, including milk products, was associated with better weight gain and significantly shorter duration of diarrhea compared with gradual introduction of feedings. Cholestyramine treatment further shortened the duration of diarrhea without adverse effects in those children who had received ORS-WHO and thus were properly rehydrated. In contrast, in children with poor initial hydration, cholestyramine treatment was associated with prolonged metabolic acidosis. We conclude that treatment of acute diarrhea by rehydration with ORS-WHO and rapid introduction of full feedings is effective and safe, and this combination forms a therapy of choice for typical hospitalized cases of acute infantile diarrhea in Finland. Cholestyramine may be of value as an adjunct therapy after adequate rehydration.