OBJECTIVE: To investigate whether vitamin D supplementation can decrease the mortality and morbidity of low birthweight infants in low income countries. DESIGN: Randomised controlled trial. SETTING: Large government hospital in New Delhi, India. PARTICIPANTS: 2079 low birthweight infants born at term (>37 weeks' gestation). MAIN OUTCOME MEASURES: Primary outcome was admission to hospital or death during the first six months of life. Main secondary outcome was growth. INTERVENTIONS: Weekly vitamin D supplements for six months at a dose of one recommended nutrient intake per day (35 µg/week). Infants were visited weekly at home for observed supplementation and were brought to the clinic monthly for clinical examination and anthropometric measurements. RESULTS: Between group differences were not significant for death or hospital admissions (92 among 1039 infants in the vitamin D group v 99 among 1040 infants in the placebo group; adjusted rate ratio 0.93, 95% confidence interval 0.68 to 1.29; P = 0.68), or referral to the outpatient clinic for moderate morbidity. Vitamin D supplementation resulted in better vitamin D status as assessed by plasma calcidiol levels at six months. In adjusted analyses, vitamin D treatment significantly increased standard deviation (z) scores at six months for weight, length, and arm circumference and decreased the proportion of children with stunted growth (length for age z score ≤ 2) or with arm circumference z scores of 2 or less. CONCLUSION: A weekly dose of vitamin D resulted in better vitamin D status and benefited the classic vitamin D function of bone growth but did not decrease the incidence of severe morbidity or death among young low birthweight infants. Trial registration ClinicalTrials.gov NCT00415402.