Several studies have established a relationship between low serum vitamin D levels and the onset of asthma in childhood. In this study, the authors aim to assess the relationship between vitamin D and asthma (1). This study included 29 mild and 30 moderate persistent asthma and 38 healthy control group. Evaluation of the three groups was carried out in respect of serum vitamin D levels, Respiratory Function Test (RFT), and Exercise Provocation Test (EPT). The two asthma groups were also examined using the Asthma Control Test (ACT) and Nitric Oxide in Exhaled Breath (FeNO) level. The results showed that the vitamin D levels of the mild and the moderate persistent asthma groups were determined to be lower than the vitamin D levels of the control group. A significant negative correlation was determined in all cases between the vitamin D levels and the broncho-reversibility percentage. The negative correlation between the vitamin D levels and the broncho-reversibility percentage was more evident in the moderate persistent asthma group. In the moderate persistent asthma group, a significant positive correlation was determined between the lowness of the maximum forced expiratory volume in EPT and a low vitamin D level. The ACT scores were lower, and the FeNO levels were higher in the moderate asthma group compared to the mild asthma group. The authors conclude that the findings showed that low serum vitamin D levels were observed more often in children with asthma, and there was a correlation with increased broncho-reversibility in the RFT and increased bronchial hyper-reactivity in the EPT.