BACKGROUND: This review was designed to assess the evidence from randomized controlled trials on effects of probiotics in the treatment and prevention of acute infectious diarrhea in infants and children. METHODS: A systematic review of published, randomized, double-blind, placebo-controlled trials on probiotics in the treatment or prevention of acute diarrhea defined as >3 loose or watery stools per 24 hours in infants and children. RESULTS: The use of probiotics as compared with placebo was associated with a significantly reduced risk of diarrhea lasting >3 days. The pooled estimate risk was 0.43 (95% CI, 0.34-0.53) with a fixed-effect model, and remained significant in a random-effect model (0.40; 95% CI, 0.28-0.57). Only Lactobacillus GG showed a consistent effect. Probiotics significantly reduced the duration of diarrhea when compared with placebo, particularly in rotaviral gastroenteritis-the pooled, weighted, mean difference (WMD) assuming the random-effect model was -20.1 hours (95% CI, -26.1 to -14.2) and -24.8 (95% CI, -31.8 to -17.9) respectively. A meta-analysis of the prevention studies was not feasible because of significant clinical and statistical heterogeneity. CONCLUSIONS: There is evidence of a clinically significant benefit of probiotics in the treatment of acute infectious diarrhea in infants and children, particularly in rotaviral gastroenteritis. Lactobacillus GG showed the most consistent effect, although other probiotic strains may also be effective. Further research is needed. Clinical and statistical heterogeneity of the prophylactic interventions preclude drawing firm conclusions about the efficacy of probiotics in preventing acute gastroenteritis.