By tradition, milk is frequently replaced by yogurt in children with diarrhoea. The beneficial effect of this feeding regime has been confirmed in persistent diarrhoea, in some cases of chronic diarrhoea and in the prevention of diarrhoea at weaning. The aim of this study was to evaluate the clinical relevance of this tradition in acute diarrhoea, with a special interest in rotavirus infection known to alter intestinal absorption. Between July 1993 and September 1995, a total of 112 Algerian children between 3 and 23 months of age were randomly assigned, after appropriate rehydration, to receive an infant formula (group IF, n=56) or yogurt made with the same IF fermented with Lactobacillus bulgaricus and Streptococcus thermophilus (group Y, n=56). Both formulas were comparable in terms of lactose concentration (40 to 42 g/l), pH (4.5), flavor (banana) and texture (addition of 3% corn starch in infant formula). Both groups were stratified according to the presence of rotavirus in stools. The consumption of oral rehydration solution (ORS), water and formulas, stool number and consistency, and duration of diarrhoea were measured. The statistical analysis (parametric and non-parametric) was carried out on all the included children. Results: Prior to inclusion in the study, children in both groups were initially similar with regards to age, sex, previous episodes of diarrhoea, type of feeding, presence of fever, and treatment with ORS and drugs. Rotavirus was found in 37 and 35 cases, in group Y and IF respectively. Formula intake (103 ± 7 ml/kg 24hrs in group IF and 110 ± 10 in group Y) and water + ORS intake (98 ± 9 ml/Kg 24hrs in IF and 128 ± 13 in Y group) were similar in both groups. The mean duration of diarrhoea was reduced from 65 ± 5 hours in group IF to 44 ± 5 in group Y (P<0.01). At 48 hours the probability of resolving the acute diarrhoeal conditions was significantly greater in patients receiving Y (64%) than in those on IF (35%; P<0.01). The beneficial effect was more apparent in group of patients with rotavirus (68% in group Y vs 27% in group IF, P<0.01). In conclusion, in young children with acute watery diarrhoea, yogurt compared to infant formula feeding significantly and markedly reduced the duration of liquid stool excretion. The beneficial effect of yogurt was more pronounced in children infected with rotavirus.