Background. Previous studies have shown that oral administration of lactobacilli can be an efficient approach to treat lactational infectious mastitis. In this trial, we have evaluated the potential of Lactobacillus salivarius PS2 to prevent this condition when orally administered during late pregnancy to women who had experienced infectious mastitis after previous pregnancies. Methods. In this study, 108 pregnant women were randomly assigned to one of 2 groups. Those in the probiotic group (n

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... 55) ingested daily 9 log 10 colony-forming units of L. salivarius PS2 from approximately week 30 of pregnancy until delivery, whereas those in the placebo group (n = 53) received a placebo. The occurrence of mastitis was evaluated during the first 3 months after delivery. Results. Globally, 44 of 108 women (41%) developed mastitis; however, the percentage of women with mastitis in the probiotic group (25% [n = 14]) was significantly lower than in the control group (57% [n = 30]). When mastitis occurred, the milk bacterial counts in the probiotic group were significantly lower than those obtained in the placebo group. Conclusions. Oral administration of L. salivarius PS2 during late pregnancy appears to be an efficient method to prevent infectious mastitis in a susceptible population. Clinical Trials Registration. NCT01505361.