Background: Vitamin D deficiency is an epidemiological problem considered as a risk factor for diseases. Vitamin D deficiency may be a factor involved in of Hashimoto's thyroiditis (HT)pathogenesis. Study Aim : assessment of vitamin D impact on the course of HT in patients of Holy Cross Region.Material and Methods: We examined 310 adults: 155- HT, 155 healthy ones. The HT was divided into subgroups: H – hypothyroid, treated with l-thyroxine (N = 110), E – euthyroid, not treated (N = 45). In 71 HT’s patients it was rated the mutation in HLA-DRB1.Results: The median of 25OHD3 in HT was significantly lower – 23,20 ng/mL (18,60-29,00) than in the control – 25,60 ng/mL (21,01-31,35) (p <0,001), but not in the E subgroup - 28,7 (21,4-33,2). There was a negative correlation of 25OHD3 with duration of the HT (p <0,001), but no with time <2 years. In the H - the median of 25OHD3 was significantly lower – 21.50 ng/ml (17.80-27.30) than in the E – 28,7 ng/mL (21,40-33,02) (p <0.001 ). In patients (-)a/TPO/a/Tg, the median of 25OHD3 was significantly higher – 32 (25,63-37,68)/28.7 (23,20-32,70) ng/ml, than in (+)/TPO/a/Tg – 22,1 (18,1-28,4)/20.2(15,4-23,6) ng/ml (p <0,001). The allele DRB1*11:04 was significantly more frequent and twice increased the risk of HT; DRB1*15:01 was significantly lower and twice reduced the risk. The proper concentration of 25OHD3 was more frequent at the presence of DRB1*13:03.Conclusions: It was shown high prevalence of vitamin D deficiency in patients with HT and may have an adverse effect on the course of the disease. This justifies the indications for the evaluation of the 25OHD3 concentration in patients with HT.