Lactic acid bacteria and bifidobacteria are increasingly being administered to pregnant women and infants with the intention of improving health. Although these organisms have a long record of safe use, it is important to identify any adverse effects in potentially vulnerable populations. In a randomized, double-blinded, placebo-controlled trial, we evaluated the safety of a bacterial dietary supplement for the prevention of atopy in infants. Two strains of lactobacilli (Lactobacillus salivarius CUL61 and Lactobacillus paracasei CUL08) and bifidobacteria (Bifidobacterium animalis subsp. lactis CUL34 and Bifidobacterium bifidum CUL20) with a total of 1 x 10(10) colony-forming units were administered daily to women during the last month of pregnancy and to infants aged 0-6 mo. Adverse events (AE) were classified according to WHO International Statistical Classification of Diseases criteria. Common symptoms were recorded by regular questionnaires. Baseline characteristics of 220 mother-infant dyads in the treatment and 234 in the placebo group were similar. Compliance with the trial interventions, loss to follow-up, symptoms, drug usage, infant growth, method of feeding, visits to the doctor, and mothers' assessment of infant health were similar in the 2 groups. Fifteen (6.8%) mothers and 73 (33.2%) infants in the treatment group and 21 (9.0%) mothers and 75 (32.1%) infants in the placebo group reported AE (P = 0.49 and P = 0.84, respectively). Severe AE occurred in 18 mothers and 63 infants with a similar frequency in each group. None of the AE were attributed to the intervention. Our findings support the safe use of this consortium of organisms during pregnancy and early infancy.