Background: Vitamin D has a role in regulating immune function, and its deficiency is a suggested risk factor for childhood pneumonia. Our aim was to assess whether oral supplementation of vitamin D3 (cholecalciferol) will reduce the incidence and severity of pneumonia in a high-risk infant population. Methods: We did a randomised placebo-controlled trial to compare oral 100 000 IU (2·5 mg) vitamin D3 with placebo given to children aged 1-11 months in Kabul, Afghanistan. Randomisation was by use of a computer-generated list. Vitamin D or placebo was given by fieldworkers once every 3 months for 18 months. Children presenting at the study hospital with signs of pneumonia had their diagnosis confirmed radiographically. Our primary outcome was the first or only episode of radiologically confirmed pneumonia. Our analysis was by intention to treat. This study is registered with ClinicalTrials.gov, number NCT00548379. Findings: 1524 children were assigned to receive vitamin D3 and 1522 placebo. There was no significant difference between the incidence of first or only pneumonia between the vitamin D (0·145 per child per year, 95 CI 0·129-0·164) and the placebo group (0.137, 0·121-0·155); the incidence rate ratio was 1·06 (95 CI 0·89-1·27). From 652 children during five separate periods of testing serum calcifediol, only one child in each of two testing periods had results greater than 375 nmol/L in the intervention group - a toxic level. Interpretations: Quarterly bolus doses of oral vitamin D 3 supplementation to infants are not an effective intervention to reduce the incidence of pneumonia in infants in this setting. Funding: Wellcome Trust and British Council.