Calcitriol (1α,25-dihydroxyvitamin D3) is the active vitamin D metabolite and mediates immunological functions, which are relevant in allergy. Its therapeutic use is limited by hypercalcaemic toxicity. We have previously shown that the activation of the vitamin D receptor inhibits IgE production and that B cells can synthesize calcitriol from its precursor 25-hydroxyvitamin D3 (inactive precursor) [25(OH)D] upon antigenic stimulation. In this study, we address the impact of 25(OH)D on the development of type I sensitization and determine its role in allergen-specific immunotherapy. BALB/c mice were sensitized to OVA, under 25(OH)D-deficient or sufficient conditions. The humoral immune response over time was measured by ELISA. OVA-specific immunotherapy was established and studied in a murine model of allergic airway inflammation using lung histology, pulmonary cytokine expression analysis, and functional parameters in isolated and perfused mouse lungs. In 25(OH)D-deficient mice, OVA-specific IgE and IgG1 serum concentrations were increased compared with control mice. OVA-specific immunotherapy reduced the humoral immune reaction after OVA recall dose-dependently. Coadministration of 25(OH)D in the context of OVA-specific immunotherapy reduced the allergic airway inflammation and responsiveness upon OVA challenge. These findings were paralleled by reduced Th2 cytokine expression in the lungs. In conclusion, 25(OH)D deficiency promotes the development of type I sensitization and correction of its serum concentrations enhances the benefit of specific immunotherapy.