Allergen specific immunotherapy (AIT) can provide long-term alleviation of symptoms for allergic disease but is hampered by suboptimal efficiency. We and others have previously shown that 1,25(OH)2-VitaminD3 (VitD3) can improve therapeutic efficacy of AIT. However, it is unknown whether VitD3 supplementation has similar effects in sublingual and subcutaneous immunotherapy. Therefore, we aimed to test VitD3 supplementation in both grass pollen (GP) subcutaneous-IT (SCIT) and sublingual-IT (SLIT) in a mouse model for allergic airway inflammation. To this end, GP-sensitized BALB/c mice received GP-SCIT or GP-SLIT with or without 10 ng VitD3, followed by intranasal GP challenges and measurement of airway hyperresponsiveness (AHR) and inflammation. VitD3 supplementation of GP-SCIT resulted in enhanced induction of GP-specific (sp)-IgG2a and suppression of spIgE after challenge. In addition, eosinophil numbers were reduced and levels of IL10 and Amphiregulin were increased in lung tissue. In GP-SLIT, VitD3 supplementation resulted in enhanced sp-IgG2a levels in serum, enhanced suppression of eosinophils and increased IL10 levels in lung tissue, as well as suppression of AHR to methacholine. These data show that VitD3 increases efficacy of both SCIT and SLIT, by enhancing induction of blocking antibodies and suppression of airway inflammation, underscoring the relevance of proficient VitD3 levels for successful AIT. Successful allergen-specific immunotherapy (AIT) induces an immunologic state of tolerance towards allergens and represents a disease-modifying treatment for allergic airway diseases, such as asthma and rhinitis 1,2. AIT is a unique form of therapy wherein allergens are administered via the subcutaneous (SCIT) or sublingual route (SLIT) to render long term relief of symptoms 3,4. The immunological mechanisms include early mast cell and basophil desensitization, inhibition of eosinophilic inflammation, induction of blocking antibodies, suppression of numbers and activity of allergen-specific Th2 cells and type 2 innate lymphoid cells (ILC2s), increases in regulatory T cells (Treg) and their associated cytokines, such as IL10 and TGF-β1 3-5. However, AIT regimes are hampered by low efficacy to suppress some clinical features of allergic airway inflammation, such as bronchial hyperresponsiveness. Moreover, the use of allergen vaccines with IgE-crosslinking capacity has safety concerns 6. To overcome the urgent need for improved AIT efficacy, ideally at lower allergen doses, several strategies have been explored 7 , including the use of adjuvants. We have previously shown the successful use of 1,25-dihydroxy-vitamin D3 (VitD3) as an adjuvant for AIT in the classical mouse model of ovalbumin-induced allergic airway inflammation 8. The physiologically active form of VitD3 binds to the VitD3 receptor (VDR), a nuclear hormone receptor, to exert its immunoregulatory