OBJECTIVES To determine whether (i) supplementation of oral 100,000 iu of vitamin D(3) (cholecalciferol) along with antibiotics will reduce the duration of illness in children with pneumonia; (ii) supplementation will reduce the risk of repeat episodes. METHODS Double-blind individually randomised placebo-controlled trial in an inner-city hospital in Kabul, of 453 children aged 1-36 months, diagnosed with non-severe or severe pneumonia at the outpatient clinic. Children with rickets, other concurrent severe diseases, very severe pneumonia or wheeze, were excluded. Children were given vitamin D(3) or placebo drops additional to routine pneumonia treatment. RESULTS Two hundred and twenty-four children received vitamin D(3;) and 229 received placebo. There was no significant difference in the mean number of days to recovery between the vitamin D(3) (4.74 days; SD 2.22) and placebo arms (4.98 days; SD 2.89; P = 0.17). The risk of a repeat episode of pneumonia within 90 days of supplementation was lower in the intervention (92/204; 45%) than the placebo group [122/211; (58%; relative risk 0.78; 95% CI 0.64, 0.94; P = 0.01]. Children in the vitamin D(3) group survived longer without experiencing a repeat episode (72 days vs. 59 days; HR 0.71; 95% CI 0.53-0.95; P = 0.02). CONCLUSION A single high-dose oral vitamin D(3) supplementation to young children along with antibiotic treatment for pneumonia could reduce the occurrence of repeat episodes of pneumonia.