Background: Despite preliminary evidence, the role of probiotics in allergy prevention is unclear.Objective: To determine whether early probiotic supplementation prevents allergic disease in high-risk infants.Methods: Newborns of women with allergy (n = 231) received either Lactobacillus acidophilus (LAVRI-A1) or placebo daily for the first 6 months of life. Children were assessed for atopic dermatitis (AD) and other symptoms at 6 and 12 months and had allergen skin prick tests (SPT) at 12 months of age.Results: A total of 178 infants completed the supplementation period. Those in the probiotic group showed significantly higher rates of Lactobacillus colonization (P =.039). At 6 months, AD rates were similar in the probiotic (n 23/89; 25.8%) and placebo (n = 20/88; 22.7%) groups (P.629). There was also no difference at 12 months, although the proportion of children with SPT+AD was significantly higher in the probiotic group (P =.045). At 12 months, the rate of sensitization was significantly higher in the probiotic group (P =.030). The presence of culturable Lactobacilli or Bifidobacterium in stools in the first month of life was not associated with the risk of subsequent sensitization or disease; however, the presence of Lactohacillus at 6 months of age was associated with increased risk of subsequent cow's milk sensitization (P =.012).Conclusion: Early probiotic supplementation with L acidophilus did not reduce the risk of AD in high-risk infants and was associated with increased allergen sensitization in infants receiving supplements. The long-term significance of the increased rate of sensitization needs to be investigated in further studies.Clinical implications: These findings challenge the role of probiotics in allergy prevention.