Background: Asthma is a heterogeneous disease characterised by chronic inflammatory airways, and is affected by several immunological factors. One of the most discussed and researched hypotheses is the relationship between vitamin D serum levels with asthma. This study aimed to investigate the relationship between vitamin D serum levels with asthma and pulmonary functions in children in Kurdistan province, Iran. Materials and methods: In this case-control study, 120 children ranging from 6 to 18 years were referred in summer for investigation. Participants were divided into two groups: asthma group, N=60; and control group, N=60. After serum separation, samples were analysed using vitamin D ELISA kit. Additionally, pulmonary function test and serum IgE levels were measured in both groups. Data were analysed using Chi-square test and multiple regression analysis in SPSS15. Results: No difference was shown between the groups in terms of gender composition (male: 57.5; female: 42.5) (P>0.05). Average Vitamin D serum level in the case group (17.98 +/- 8.68) was less than in control group (22.35 +/- 6.26) (P<0.01). In addition, the difference of vitamin D deficiency in level of suboptimal between the asthma (17.77 +/- 6.41) and the control group (24.9 +/- 3.18) was statistically significant (P<0.0001). Positive correlation existed between vitamin D levels with FEV1, FVC, and FEV1/FVC. Multiple regression analysis showed a reverse relationship between vitamin D levels with IgE serum levels; this remained after adjustment for potential confounders (e.g. age, sex, BMI, FEV1, and FVC). Conclusion: The results showed that serum levels of vitamin D in asthma patients were less than in healthy people, and also reduced lung function in these patients. So, the serum levels of vitamin D in asthma patients must be checked continuously. (C) 2016 SEICAP Published by Elsevier Espana, S.L.U. All rights reserved.