Many large intervention trials have shown that high-dose vitamin A supplementation decreases overall childhood mortality rates by 23-30%, making it one of the most cost-effective interventions in low-income countries. However, with regard to morbidity and the immunological mechanisms behind the beneficial effect on mortality rates, the results are less clear. Furthermore, vitamin A seems to be beneficial in some but not all age groups, and smaller doses may be even better than higher doses. It seems likely that by pursuing these inconsistent observations, vitamin A supplementation programmes can be optimised.