Objective - To study the effect of vitamin A supplementation on morbidity and mortality from infectious disease. Design - A meta-analysis aimed at identifying and combining mortality and morbidity data from all randomised controlled trials of vitamin A. Results - Of 20 controlled trials identified, 12 trials were randomised trials and provided 'intention to treat' data: six community trials in developing countries, three in children admitted to hospital with measles, and three in very low birth weight infants. Combined results for community studies suggest a reduction of 30% (95% confidence interval 21% to 38%; two tailed p < 0.0000001) in all cause mortality. Analysis of cause specific mortality showed a reduction in deaths from diarrhoeal disease (in community studies) by 39% (24% to 50%; two tailed p < 0.00001); from respiratory disease (in measles studies) by 70% (15% to 90%; two tailed p = 0.02); and from other causes of death (in community studies) by 34% (15% to 48%; two tailed p = 0.001). Reductions in morbidity were consistent with the findings for mortality, but fewer data were available. Conclusions - Adequate supply of vitamin A, either through supplementation or adequate diet, has a major role in preventing morbidity and mortality in children in developing countries. In developed countries vitamin A may also have a role in those with life threatening infections such as measles and those who may have a relative deficiency, such as premature infants.