OBJECTIVES: To determine whether subclinical vitamin D deficiency in Indian children under 5 y of age is a risk factor for severe acute lower respiratory infection (ALRI). DESIGN: A hospital-based case-control study. SETTING: Sanjeevani Paediatrics Hospital, a private hospital in Indapur, India. PARTICIPANTS: A total of 150 children including 80 cases and 70 controls, aged 2-60 months, were enrolled. Case definition of severe ALRI as given by the World Health Organization was used for cases. Controls were healthy children attending outpatients' service for immunization. MAIN OUTCOME MEASURE: Association of serum 25-hydroxyvitamin D3 (25OHD3) with severe ALRI, controlling for demographic and other potential risk factors. RESULTS: Serum 25OHD3 increased with age. Factors significantly associated with decreased risk of severe ALRI in univariate analysis were: exclusive breastfeeding in the first 4 months (cases 35/78 (45%), controls 41/64 (64%); P=0.02); introduction of other dietary liquids than milk only after 6 months (cases 46/70 (66%), controls 31/66 (47%); P=0.03); use of liquid petroleum cooking fuel (cases 32/80 (40%), controls 40/70 (57%); P=0.04); infant not covered in swaddling cloths when exposed to sunlight before crawling (cases 11/52 (21%), controls 25/54 (46%); P=0.006); and serum 25OHD3>22.5 nmol/l (cases 16/80 (20%), controls 48/70 (69%); P<0.001). In multivariate analysis, factors associated with significantly lower odds ratio for having severe ALRI were: serum 25OHD3>22.5 nmol/l (OR: 0.09; 95% CI 0.03-0.24; P<0.001) and exclusive breastfeeding in the first 4 months of life (OR 0.42; 95% CI 0.18-0.99; P=0.046) with age and height/age as significant covariates. CONCLUSION: Subclinical vitamin D deficiency and nonexclusive breastfeeding in the first 4 months of life were significant risk factors for severe ALRI in Indian children.