BACKGROUND: It is difficult to permanently change the composition of the complex intestinal microflora of the adult. Orally administered probiotic bacteria produce only temporary colonization of the intestine in patients with a fully developed gut microflora. The gastrointestinal tract of a healthy fetus is sterile. During the birth process and rapidly thereafter, microbes from the mother and the surrounding environment colonize the gastrointestinal tract until a dense, complex microflora develops. Probiotic bacteria have been shown to beneficially influence the intestinal and systemic immune system and mediate protection against nosocomial infections affecting the neonate. OBJECTIVES: The purpose of this study was to determine whether oral administration of the probiotic micro-organism Lactobacillus rhamnosus strain GG (L. GG) to the pregnant woman leads to colonization of the newborn infant. METHODS: The authors identified six women who were taking L. GG during late pregnancy. None of the children received L. GG after birth, and their mothers discontinued its consumption at the time of delivery. L. GG concentration in fecal samples was determined by colony morphology and molecular analysis. RESULTS: In all four children delivered vaginally and in one of two children delivered by cesarean section, L. GG was present in fecal samples at 1 and 6 months of age. Three children remained colonized for at least 12 months, and in two children L. GG was detected in fecal samples at 24 months of age. Three mothers were tested 1 month post partum and no L. GG was present in fecal samples. No L. GG was found in one of these women 24 months post partum. There was no L. GG detectable in stools of the siblings of two children at the 2-year and 3-years after birth of the index child. L. GG was not isolated from the stools of children whose mothers were not taking L. GG. CONCLUSIONS: Temporary colonization of an infant with L. GG may be possible by colonizing the pregnant mother before delivery. Colonization is stable for as long as 6 months, and in unexplained circumstances may persist for as long as 24 months.