Bronchiolitis refers to a condition that is associated with nonspecific inflammatory injury to the small airway, often sparing a considerable portion of the interstitium. Until now, the prevalence, mortality, comorbidities, and medical expenditure for bronchiolitis have not been elucidated. We used sample data of KNHIS-NSC (Korea National Health Insurance Service-National Sample Cohort), which is a stratified random sample of 50 million Koreans considering age, sex, residence area, income, etc. Inclusion criteria were: 1) patients with ICD-10 Code J448 or J684, 2) patients confirmed by CT scan, 3) patients over 20 years old, and 4) patients who visited clinic or hospital in South Korea from 2002 to 2013. We excluded patients diagnosed with acute bronchiolitis (ICD-10 Code J21) or cystic fibrosis (ICD-10 Code E84). The overall prevalence of bronchiolitis was 688 cases per 1,000,000 population. In terms of comorbidities, rhinitis (52.3%) was the most common, followed by asthma (52.2%) HTN (43.7%), GERD (30.6%), chronic rhinosinusitis (28.7%), DM (22.8%) and osteoporosis (17.9%) (Graph 1). Other comorbidities were cerebrovascular disease (16.9%), angina (14.4%) and peripheral vascular disease (13.4%). Healthcare spending on bronchiolitis averaged about 465,000 won (350 euros) per person per year. The mortality rate was the highest due to malignancy (512 cases, 6.2%), followed by bronchiolitis (270 cases, 3.3%), cerebrovascular disease (90 cases, 1.1%), and pneumonia (68 cases, 0.8%). Among the malignant diseases, lung or bronchi cancers (229 cases, 2.8%) accounted for about half of all cases. In Conclusion, bronchiolitis is an underestimated, burdensome disorder with various medical comorbidities, and further study should be carried out for the advancement of its concept.