Short title Effect on Caries in Primary Teeth after Administration of L. reuteri Declaration of Conflicts AbstractThe aim of this study was to evaluate the effect, on oral health at age 9 years, of daily oral supplementation with the probiotic Lactobacillus reuteri, strain ATCC 55730 to the mothers during the last month of gestation and to the children through the first year of life. The study comprised a single-blind, placebo-controlled, multicenter trial involving 113 children: 60 in the probiotic and 53 in the placebo group. The subjects underwent clinical and radiographic examination of the primary dentition, carious lesions, plaque and gingivitis were recorded.Saliva and plaque were sampled, for determination of mutans streptococci (MS) and lactobacilli (LB) in saliva and plaque as well as salivary secretory IgA (SIgA). Forty-nine (82%) of the children in the probiotic group and 31 (58%) in the placebo group were caries free (p < 0.01). The prevalence of approximal caries lesions was lower in the probiotic group (0.67 ± 1.61 vs. 1.53 ± 2.64; p < 0.05) and there were fewer sites with gingivitis compared to the placebo group (p < 0.05). There were no significant differences between the groups with respect to frequency of tooth-brushing, plaque and dietary habits but to intake of fluoride supplements (p < 0.05). There were no intergroup differences with respect to L. reuteri, MS, LB or SIgA in saliva. Within the limitation of this study it seems that daily supplementation with L. reuteri from birth and during the first year of life is associated with reduced caries prevalence and gingivitis score in the primary dentition at 9 years of age.