Although vitamin A deficiency in children seems to increase susceptibility to infection and community trials have shown that vitamin A supplementation can reduce childhood mortality from infectious diseases, the underlying biological mechanisms are largely unknown. We conducted a randomised, double-masked, placebo-controlled clinical trial among children in West Java, Indonesia, to determine whether vitamin A deficiency is associated with abnormalities in T-cell subsets and whether vitamin A supplementation affects T-cell subsets. We studied 55 children aged 3-6 years--30 with xerophthalmia and 25 without. Acutely malnourished children (< 80% of reference weight-for-height) were excluded. CD4/CD8 ratios and the proportions of circulating CD4 naive, CD4 memory, CD8, CD45RA, and CD8, CD45RO T-cell subsets were measured. Children with xerophthalmia had lower CD4/CD8 ratios (p < 0.08), lower proportions of CD4 naive T cells (p < 0.03), and higher proportions of CD8, CD45RO T cells (p < 0.04) than those without xerophthalmia. 26 children were given vitamin A supplementation (60 mg retinol equivalent) and 29 received placebo. 5 weeks later the vitamin A group had higher CD4/CD8 ratios (p < 0.001), higher proportions of CD4 naive T cells (p < 0.01), and lower proportions of CD8, CD45RO T cells (p < 0.05) than the placebo group. Vitamin-A-deficient children have underlying immune abnormalities in T-cell subsets and these abnormalities are reversible with vitamin A supplementation.