Aim: Certain probiotic agents, e.g. Lactobacillus GG, have shown efficacy in clinical trials for the treatment of acute childhood diarrhoea, but few studies have examined the effect of Saccharomyces boulardii. We evaluated the effect of S. boulardii in children with acute diarrhoea. Methods: Two hundred children were randomized to receive S. boulardii in a granulated form in a daily dose of 250 mg (S. boulardii group) or placebo (placebo group) for 5 d. Clinical and demographic characteristics on admission were similar between the study groups. Results: The medians of the average stool frequency after the second day of the treatment were significantly lower in the S. boulardii group than in the placebo group (p=0.003). The duration of diarrhoea significantly reduced in the S. boulardii group compared with the placebo group (4.7 vs 5.5 d, p = 0.03). The effect of S. boulardii on watery diarrhoea became apparent after the second day of the treatment. The duration of hospital stay was shorter in the S. boulardii group than in the placebo group (2.9 vs 3.9 d, p<0.001). Four children from the placebo group versus only one child from the S. boulardii group had persisting diarrhoea.