Background: Various populations of regulatory T cells play a central role in the development of peripheral tolerance to allergens. Culturing of CD4+ T cells isolated from peripheral blood of allergic patients with vitamin D induces the generation of stable IL-10 producing CD4+CD25+ Treg cells suppressing the proliferation of T helper cells obtained from the same patients. The immune regulatory role of vitamin D in allergic patients has been controversial and obviously needs a more clarifying research work. Aim of the work: to determine the percentage of induced T regulatory cells producing interleukin 10 after stimulation of T regulatory cells with cow milk allergen in the presence of vitamin D in culture. This aims to further in-vitro study the immune regulatory role of vitamin D in cow milk allergic patients. Results: there is association between decreased level of vitamin D and milk-allergy, as serum level of 25(OH) D3 was insufficient in 16 (80 %) patients (10- 29.9 ng/ml) while 4 (20%) patients were sufficient (30-100 ng/ml). Addition of  vitamin D, in culture, induces the production of CD4+ CD25hi Foxp3+ IL10+    . Treg cells within peripheral blood mononuclear cells (PMNCs) isolated from allergic children who had insufficient vitamin D, but not in allergic children who had normal level of vitamin D. Conclusion: this work provides further evidence for an important role of 1,25(OH)2D3 as an immune-modulatory molecule and suggests that supplementation of vitamin-D-deficient individuals, who are reported to have reduced numbers of circulating and Foxp3⁺ IL10⁺ Treg cells, may represent an attractive therapy for enhancing endogenous populations of Treg cells in allergy.