Probiotics, defined as microbial cell preparations or components of microbial cells that have a beneficial effect on the health and well being of the host, have traditionally been used to treat and prevent a variety of infections. Beneficial effects of probiotics in acute infectious diarrhea in children seem to be: (i) moderate; (ii) strain-dependent; (iii) dose dependent; (iv) significant in watery diarrhea and viral gastroenteritis, but non-existent in invasive, bacterial diarrhea; and (v) more evident when treatment with probiotics is initiated early in the course of disease. Three large, randomized controlled trials (RCTs) provide evidence of a very modest effect (statistically significant, but of questionable clinical importance) of some probiotic strains (Lactobaccillus GG, Lactobaccillus reuteri, Bifodobacterium lactis) on the prevention of community-acquired diarrhea. We have found conflicting evidence from four RCTs on the efficacy of Lactobacillus GG and B. bifidum and Streptococcus thermophilus in the prevention of nosocomial diarrhea in children. Two RCTs in children provide evidence of a moderate beneficial effect of Lactobacillus GG in the prevention of antibacterial-associated diarrhea (AAD), but results in adults are conflicting. Data on the efficacy of other probiotic strains in AAD in children are very limited. In conclusion, to date, the most extensively studied and best documented clinical application of probiotics in children is for the treatment of acute watery diarrhea of rotaviral or presumably viral etiology. Studies documenting effects in other types of diarrheal diseases in children are limited, although some preliminary results are promising. The effects of different probiotic microorganisms are not equal. Only very few probiotic strains have been tested rigorously in RCTs. Many questions remain to be answered. Future clinical trials should evaluate carefully selected, precisely defined probiotic strains and address clinically important endpoints.