BACKGROUND: Acute infectious diarrhea is a major cause of childhood morbidity and economic burden for families. We evaluate the clinical, microbiologic, and immunologic effects of probiotics in acute infectious diarrhea. METHODS: Children (n = 304) aged 3 months to 6 years hospitalized for acute diarrhea were randomized to receive Bio-three (a mixture of Bacillus mesentericus, Enterococcus faecalis, and Clostridium butyricum) or placebo orally 3 times daily for 7 days. Fecal samples were homogenized for bacterial culture and blood cells were isolated for cell culture and cytokine analysis. This study is registered (NCT00463190). RESULTS: The mean duration of diarrhea after start of therapy was 60.1 hours in the probiotics group versus 86.3 hours in the placebo group (P = 0.003). Hospital stay was shorter in the probiotics group than in the placebo group (P = 0.009). Counts of Bifidobacteria and Lactobacillus species were elevated in stool culture of the probiotics (Bio-three) group. IL-10 was increased in the serum and supernatants of cell culture in the probiotics group, and tumor necrosis factor-alpha values were down-regulated. Interferon- gamma and IL-12 were mildly elevated in the probiotics group, compared with the placebo group. CONCLUSIONS: This probiotics mixture reduced the severity of diarrhea and length of hospital stay in children with acute diarrhea. In addition to restoring beneficial intestinal flora, probiotics may enhance host protective immunity such as down-regulation of pro-inflammatory cytokines and up-regulation of anti-inflammatory cytokines.