Two prospective studies compare the dietary treatment of acute diarrhoea (gastroenteritis in infancy with either partially adapted formula or with special formula (HN 25) with a low lactose content (1.8%), or virtually free of lactose (0.05%), including only infants with acute diarrhoea, a loss in weight not exceeding 8%, and without parenteral fluid intake. There is no difference in the duration of hospitalization. However, the time until normalization of the stools was significantly shorter when lactose-reduced formula or formula virtually free of lactose was fed (4.4 resp. 3.1 vs. 5.1 days). There were 9 relapses in the control group fed partially adapted formula, and none in the trial group fed special formula. The present results show that in mild and moderate diarrhoea a dietary treatment with lactose-reduced formula should be preferred to a treatment with infant formula.