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 ¼ 0.02); introduction of other dietary liquids than milk only after 6 months (cases 46/70 (66%), controls 31/66 (47%); P ¼ 0.03); use of liquid petroleum cooking fuel (cases 32/80 (40%), controls 40/70 (57%); P ¼ 0.04); infant not covered in swaddling cloths when exposed to sunlight before crawling (cases 11/52 (21%), controls 25/54 (46%); P ¼ 0.006); and serum 25OHD3 422.5 nmol/l (cases 16/80 (20%), controls 48/70 (69%); Po0.001). In multivariate analysis, factors associated with significantly lower odds ratio for having severe ALRI were: serum 25OHD3 422.5 nmol/l (OR: 0.09; 95% CI 0.03-0.24; Po0.001) and exclusive breastfeeding in the first 4 months of life (OR 0.42; 95% CI 0.18-0.99; P ¼ 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. (2004) 58, 563-567. doi:10.1038/sj.ejcn.1601845 Keywords: pneumonia; children; India; 25-hydroxyvitamin D3; breastfeeding European Journal of Clinical Nutrition IntroductionAcute lower respiratory infection (ALRI), primarily pneumonia, is a common cause of morbidity and mortality in children younger than 5 y of age, particularly in developing countries (United State Agency for International Development (USAID), 2002). Consequently considerable research has aimed at finding effective interventions against ALRI such as immunization (Monto & Lehmann, 1998) and casemanagement involving antibiotics (World Health Organization (WHO), 1995). Micronutrient supplementation is another potential intervention. Many trials have investigated the benefits of vitamin A supplements, but these do not significantly decrease morbidity and mortality from ALRI, other than that due to measles, in spite of decreasing overall under-5 mortality (Vitamin A and Pneumonia Working Group, 1995). Zinc supplementation lowered ALRI incidence by 41% (Zinc Investigators' Collaborative Group, 1999). Contributors: VW was involved in designing the study and collected and analysed the data. SF was...