The study included 78 children: 38 patients with cystic fibrosis (CF) (median age 8.2 [4.9-13.8] years) and 40 healthy children (median age 7.66 [2.0-12.1] years), living in the Stavropol region during 2018-2019. Vitamin D sufficiency was assessed by the content of calcidiol -25(OH)D in blood plasma. Seasonal fluctuations in the level of calcidiol during the year in children with CF were more pronounced than in children from the control group -12.2 [7.6-20.2] ng/ml in winter, 29.8 [21.3-37 .9] ng/ml in spring, 33.2 [26.5-39.1] ng/ml in summer and 22.6 [11.4-30.5] ng/ml in autumn, in healthy children the level is 25(OH)D during 2018 was 34.8 [24.8-53.1] ng/ml in winter, 31. 1 [24.6-44.6] ng/ml in spring, 30.4 [23.3-35.3] ng/ml in summer and 41.9 [32.1-55.2] ng/ml in autumn. Serum calcidiol levels were significantly lower in CF children compared to the control children group in winter (p=0.007) and autumn (p=0.04). During the study, the number of children with vitamin D deficiency and severe vitamin D deficiency significantly decreased. At the beginning of the study, severe vitamin D deficiency was detected in 40 % of patients with CF; after adjusting the dose of vitamin received in spring (p<0.005) and summer (p<0.005), it was not found in any of the children. On the contrary, the frequency of normal vitamin D sufficiency increased significantly by 36.7 % in spring (p<0.05), by 50 % in summer (p<0.01), and by 16.7 % in autumn (p>0.05), compared with the first determination of 25(OH)D in winter. Thus, the frequency of vitamin D deficiency in CF patients is statistically significantly higher than in healthy children. The problem of vitamin D deficiency in children with CF is quite relevant due to the low awareness of CF patients and their parents about the critical role of cholecalciferol in this disease.