The effect of diosmectite on intestinal permeability changes in acute diarrhea was measured during a double-blind placebo-controlled trial carried out in 59 Gabonese children aged 5-35 months. Intestinal permeability tests (IPTs), measuring the urinary elimination of orally administered lactulose and mannitol at a dosage of 1 g/10 kg each, were performed during the morning following admission and repeated 2 days later after treatment by diosmectite or placebo. During the first IPT, urine volume and lactulose and mannitol urinary recoveries were comparable in the diosmectite and in the placebo groups: 50 vs. 35.5 ml (median; p = 0.21), 1.01 vs. 1.27% (p = 0.35), and 2.20 vs. 2.87% (p = 0.12). As a result, the lactulose/mannitol (L/M) ratio was similar in the two groups: 44.44 vs. 35.33% (p = 0.98). During the second IPT, the urinary lactulose recovery decreased similarly in both groups (-0.18 vs. -0.29%; p = 0.76), whereas the urinary mannitol recovery exhibited opposite variations, the increase in the diosmectite group (+ 1.43%) contrasting significantly with the decrease in the placebo group (-0.47%; p = 0.01). When comparing the first and the second IPT, the decrease of the L/M ratio was significant in the diosmectite group (44.44 vs. 28.32%; p = 0.02) and not in the placebo group (35.33 vs. 48.23%; p = 0.91). During gastroenteritis, diosmectite appears to enhance absorption of mannitol, a marker of intestinal absorptive area.