We have studied a population of 60 children with acute diarrhea. In order to evaluate the benefit of lactose suppression from the diet on the clinical course of the disease, these children were randomly assigned to either the control group or the experimental group. No differences were found in respects to the duration of the illness (lactose containing formula 4.8 +/- 2 days vs lactose free formula 4.4 +/- 3.1 days). Similarly, body weight change did not change significantly (12.5 +/- 235.8 gr vs 55.9 +/- 238.0 gr, t-test non-significant). Children fed with the lactose free diet voided less frequently than did children in the control group (12.8 +/- 10.5 vs 19.8 +/- 9.5 times; p < 0.01). This difference disappeared when only invasive diarrheas were considered (26.6 +/- 12 vs 21.5 +/- 9.5 voidings; t-test nonsignificant). Finally, only 2 of the 32 cases fed with lactose containing formula showed clinical intolerance to lactose. Therefore, we conclude that in patients with mild-to-moderate acute diarrhea, the resumption of feeding should be undertaken with the same formula that they were previously fed, restricting the use of lactose free formulas to selected cases and to those children in whom a standard milk made symptoms reappear.