Abstract: Li Yuanjun, Zhang Xinglian, Zhang Huanzhen, Zhang Xianhui, Liu Runtao, Wang Hui, Han Yuyang, Li Junna, Liu Qiang Department of Dermatology, Children′s Hospital of Shanxi, Taiyuan 030013, China （Li YJ, Zhang XL, Zhang HZ, Liu RT, Wang H, Han YY, Li JN, Liu Q）; Clinical Laboratory, Children′s Hospital of Shanxi, Taiyuan 030013, China （Zhang XH） Corresponding author: Liu Qiang, Email: lqwcj@126.com 【Abstract】 Objective To detect the serum level of vitamin D in infants with atopic dermatitis（AD）, and to investigate the relationship between the serum level of vitamin D and severity of AD in infants. Methods Clinical data were collected from patients with moderate to severe AD （AD group） through a questionnaire survey in Children′s Hospital of Shanxi from February to April in 2016, and the severity of AD was evaluated by the SCORing atopic dermatitis （SCORAD） score. A total of 95 health checkup examinees served as the control group. Enzyme-linked immunosorbent assay（ELISA）was performed to detect the serum level of 25（OH）D3 in the AD group and control group, as well as the total serum IgE level in the AD group. Blood cell analyzer was used to determine the proportion of blood eosinophils in the AD group. Results A total of 97 patients with AD were enrolled into the study, including 43 （44.3 %） patients with moderate AD and 54 （55.7%） patients with severe AD. The serum level of 25（OH）D3 was significantly lower in the AD group than in the healthy control group （［66.71 ± 21.07］ nmol/L vs.［85.43 ± 14.87］ nmol/L, P < 0.01）, as well as in the patients with severe AD than in the patients with moderate AD （［47.54 ± 29.36］ nmol/L vs.［63.89 ± 26.67］ nmol/L, P = 0.006）. The proportion of blood eosinophils was significantly higher in the severe AD group than in the moderate AD group （0.124 ± 0.094 vs. 0.061 ± 0.060, P < 0.001）. There was no significant difference in the total serum IgE level between the moderate AD group and severe AD group （P = 0.375）. Among the patients with AD, the serum level of 25（OH）D3 was negatively correlated with the proportion of blood eosinophils （r = -0.336, P < 0.05）, but there was no correlation between the serum level of 25（OH）D3 and total serum IgE level （r = -0.174, P > 0.05）. The serum level of 25（OH）D3 was significantly associated with breastfeeding and vitamin D supplementation （P < 0.05）, but unrelated to age, gender, course of disease and acute exudative phase （all P > 0.05）. Conclusion The serum level of 25（OH）D3 is evidently decreased in infants with AD, and vitamin D deficiency is closely related to the severity of AD in infancy.