OBJECTIVES: To perform an updated meta-analysis of the effect of vitamin A supplementation on childhood morbidity from respiratory tract infections and diarrhea. Study design A comprehensive search of the 1966 to 2000 MEDLINE database and review of the reference lists of relevant articles identified 9 randomized controlled trials dealing with morbidity from respiratory infections and diarrhea in children 6 months to 7 years of age and provided "intention-to-treat" data. RESULTS: The combined results indicated that vitamin A supplementation has no consistent overall protective effect on the incidence of diarrhea (relative risk, 1.00; 95% CI, 0.94-1.07) and that it slightly increases the incidence of respiratory tract infections (relative risk, 1.08; 95% CI, 1.05-1.11). CONCLUSIONS: High-dose vitamin A supplements are not recommended on a routine basis for all preschool children and should be offered only to individuals or populations with vitamin A deficiency.