Factors that influence immune regulation in early life may be implicated in the rise in allergic disease, including reduced microbial burden. The aim of the study was to examine the infant regulatory T-cell function in relation to (a) probiotic supplementation for the first 6 months of life and (b) the subsequent development of an early allergic phenotype. Two hundred and thirty-one allergic, pregnant women were recruited into a randomized, controlled trial. The infants received either a probiotic or placebo daily for the first 6 months of life. One hundred and seventy-eight children completed the study, with blood samples available from 118 (60 placebo; 58 probiotic). CD4+CD25+CTLA4+T-regulatory phenotype and allergen-induced FOXP3 mRNA expression were compared in relation to this intervention as well as according to evidence of early disease (atopic dermatitis). The administration of probiotics was not associated with any significant differences in the proportion of circulating CD4+CD25+CTLA4+cells, or in the resting expression of FOXP3. There were also no relationships between these parameters and patterns of gut colonization, and this probiotic did not reduce the risk of atopic dermatitis. Children who developed atopic dermatitis (n = 36/118) had significantly higher induced FOXP3 expression following stimulation with both house dust mite (HDM) (p = 0.017) and ovalbumin (OVA) allergens (p = 0.021) than those that did not develop atopic dermatitis. Although this relationship was seen in both the probiotic and placebo groups, it was more pronounced in the probiotic group. However, regression analysis demonstrated that higher allergen-induced FOXP3 expression was predicted by the presence of atopic dermatitis (p = 0.018) rather than probiotics supplementation (p = 0.217). The higher levels of allergen-induced FOXP3 in atopic dermatitis suggest activation of these compensatory mechanisms rather than a primary defect in this pathway. Probiotic supplementation and gut colonization did not appear to substantially modify these regulatory pathways, or the risk of developing atopic dermatitis.