Populations of T lymphocytes cells called regulatory T lymphocytes (Tregs) play a key role in the formation and maintenance of immune tolerance. This phenotypically heterogeneous group of cells regulates the immune system and plays an important role in tumor immunology, transplant immunology and the pathogenesis of multiple disorders, in particular autoimmune diseases. In the study it was decided to investigate the difference in the correlation of levels of vitamin D3 (25-OHD3) with the percentage of Tregs cells (FoxP3+) in children with no symptoms of allergy (control group), and with symptoms of allergy. In addition, in both groups the phenotype of lymphocytes was identified. The results indicate a positive correlation between Tregs and 25-OHD3 only in children with allergic symptoms (no correlation in the control group), along with a simultaneous significant reduction in the percentage of CD4 lymphocytes in this group.