Acute lower respiratory infection (ALRI), primarily pneumonia and bronchiolitis, is a substantial cause of morbidity and mortality in children younger than 5 years of age, particularly in developing countries. Worldwide, 20% mortality among children aged less than 5 years is attributed to respiratory tract infections (predominantly pneumonia associated). With neonatal pneumonia inclusive, 35-40% mortality among children aged less than 5 years account for 2.04 million deaths/year. Southeast Asia stands first in number for ARI incidence accounting for more than 80% of all incidences together with sub Saharan African countries. 1,2 In India, more than ABSTRACT Background: Acute lower respiratory infection (ALRI), primarily pneumonia and bronchiolitis, is a substantial cause of morbidity and mortality in children <5 years of age, particularly in developing countries. Vitamin D deficiency is a significant risk factor for severe ALRI in Indian infants and children <5 years of age. This study was done to determine the relationship, if any, between respiratory illnesses and serum vitamin D status. Methods: This study included 40 (22 boys and 18 girls) hospitalized children of 6 months to 5 years of age with ALRI as cases and 40 (25 boys and 15 girls) age and sex matched children with no respiratory illness and no clinical manifestation of vitamin D deficiency as controls. Sociodemographic profile, feeding history, immunization history, sun exposure, drug history, causes of admission were taken by interviewing the child's mother or guardian. Serum vitamin D was estimated using commercially available kit by chemiluminiscence technique. General physical and systemic examination with emphasis on respiratory system was performed. ALRI diagnosis was made by clinical, radiographic and biochemical findings. Results: Among vitamin D deficient and insufficient children, 71% and 74% were adequately exposed to sunlight respectively. 50% vitamin D deficient children were diagnosed of bronchial asthma and 33% of bronchopneumonia. Among vitamin D insufficient, 48% had acute bronchiolitis and 35% had bronchopneumonia. Among sufficient Vitamin D, bronchopneumonia and acute bronchiolitis were 36% each i.e. most of the cases of bronchial asthma (60%) had vitamin D deficiency whereas majority of acute bronchiolitis cases (73%) had vitamin D insufficiency. Statistically significant difference was seen between cases and control among vitamin D sufficient, insufficient and deficient groups with higher number of sufficient vitamin D controls. Low vitamin D levels were significantly correlated with ALRI. Conclusions: Subclinical vitamin D deficiency is significant risk factors for severe ALRI in Indian children of less than 5 years of age.