Abstract Background There is conflicting evidence about the association between low vitamin D levels and asthma. Aim The aim of this work was to assess the relation between the serum level of vitamin D and the asthma severity. Patients and method A total of 90 asthmatic patients were diagnosed of bronchial asthma clinically and functionally according to GINA guidelines classification 2012. The serum 25-hydroxy vitamin D level was measured in all the cases. The relation between the serum level of vitamin D and forced expiratory volume at 1 s (FEV1) was analyzed. Then, it was correlated to age, sex, and BMI variables. Results Patients were divided into three groups according to FEV1 prebronchodilators. The mean value of vitamin D was 30.43 in mild cases, whereas it was 20.27 in moderate cases, and 11.97 in severe cases. These data stated that the lower the FEV1, the greater the vitamin D deficiency levels, and this relation was statistically highly significant (P