Infants aged 4 to 36 months with acute diarrhoea (rotavirus 66%) were treated as outpatients with oral fluids and a rapid return to full feedings. In addition, the infants were randomized to receive for 3 days either cholestyramine 2 g twice daily (N = 10), an equivalent placebo 2 g twice daily (N = 15), or loperamide 0.10 mg/kg divided in three doses (N = 16). The duration of watery diarrhoea from the beginning of treatment was 0.9 +/- 1.0 days in the cholestyramine group, 2.5 +/- 1.3 days in the loperamide group, and 3.3 +/- 1.6 days in the placebo group (p less than 0.001 cholestyramine vs. placebo, p less than 0.005 cholestyramine vs. loperamide). The infants receiving cholestyramine also had a better weight gain than those receiving the placebo, and their metabolic acidosis was corrected sooner. There was no hyperchloraemia associated with the cholestyramine treatment. It is concluded that cholestyramine 2 g twice daily for 3 days can be safely used to shorten the course of acute diarrhoea. The use of loperamide in acute infantile diarrhoea does not appear justified.