Dioctahedral smectite (DS) a natural adsorbent clay capable of adsorbing viruses, bacteria, and other intestinal irritants in vitro, is claimed to possess beneficial "antidiarrheal" properties. This study tested the effect of DS on the duration of diarrhea and the frequency and amount of liquid stools. Ninety well-nourished boys, aged 3-24 months, with acute watery diarrhea and mild, moderate, or severe dehydration were included in a randomized double-blind, placebo-controlled trial. After initial rehydration, they received DS or placebo (1.5 g freshly dissolved in 50 ml of water, four times daily for 3 days) along with oral rehydration solution (ORS) and adequate feeding. The clinical characteristics of both groups were comparable on admission. Patients in the smectite group had a significantly shorter duration of diarrhea (mean +/- SD, 54 +/- 16 vs. 73 +/- 13 h) and significantly fewer stools (2.6 +/- 0.8 vs. 3 +/- 0.7 on second day; 1.9 +/- 0.7 vs. 2.4 +/- 0.7 on third day; and 11.3 +/- 3.2 vs. 13.8 +/- 3 overall). The amount of liquid stools was not significantly reduced. Weight gain at 24, 48, and 72 h and on recovery was significantly higher in the smectite group despite the comparable fluid and food intake in both groups. These results suggest a beneficial effect of DS in shortening the duration of diarrhea and reducing the frequency of liquid stools in children rehydrated with ORS.