Antibiotic use is considered among the most severe causes of disturbance to children's developing intestinal microbiota, and frequently causes adverse gastrointestinal effects ranging from mild and transient diarrhoea to life- Threatening infections. Probiotics are commonly advocated to help in preventing antibiotic- Associated gastrointestinal symptoms. However, it is currently unknown whether probiotics alleviate the antibiotic- Associated changes in children's microbiota. Furthermore, it is not known how long- Term probiotic consumption influences the developing microbiota of children. We analysed the influence of long- Term Lactobacillus rhamnosus GG intake on preschool children's antibiotic use, and antibiotic- Associated gastrointestinal complaints in a double blind, randomized placebo-controlled trial with 231 children aged 2-7. In addition, we analysed the effect of L. rhanmosus GG on the intestinal microbiota in a subset of 88 children. The results show that long- Term L. rhamnosus GG supplementation has an influence on the composition of the intestinal microbiota in children, causing an increase in the abundance of Prevotella, Lactococcus, and Ruminococcus, and a decrease in Escherichia. The treatment appeared to prevent some of the changes in the microbiota associated with penicillin use, but not those associated with macrolide use. The treatment, however, did reduce the frequency of gastrointestinal complaints after a macrolide course. Finally, the treatment appeared to prevent certain bacterial infections for up to 3 years after the trial, as indicated by reduced antibiotic use.