Purpose: Vitamin D plays an important role in calcium homeostasis and bone metabolism. And it is associated with various diseases such as cardiovascular, immune, allergic and infectious disease. The aim of this study was to investigate the difference in clinical manifestations according to the concentration of vitamin D in mild bronchiolitis. Methods: We performed a retrospective review of medical records of patients with mild bronchiolitis from November 2016 to April 2017 in Daegu Fatima Hospital. Mild bronchiolitis was classified by the modified Tal’s score method. Patients were divided into two groups according to 25(OH)-vitamin D levels by 20 ng/ml. We analyzed the clinical characteristics and laboratory data of the two groups. Results: Of the 64 patients, 19 were included in the deficiency group and 45 in the normal group. Vitamin D level was 11.7 ± 4.9 ng/ml in the deficiency group and 28.8 ± 5.0 ng/ml in the normal group. There were no differences in age, sex, weight, birth history, vitamin D supplements, and family environment, clinical features in both groups. But, vitamin D deficiency group had a significantly longer hospitalization time than normal group(6.78 ± 2.74 days vs 5.3 ± 1.7days, P=0.45). In the deficiency group, respiratory disease morbidity was significant higher ( P = 0.001). No significant difference in blood and respiratory virus tests was observed. Conclusion: Low vitamin D levels in mild bronchiolitis are associated with a longer hospitalization time and prior respiratory disease. Vitamin D may affect the course of mild bronchiolitis