Background and objective: In developing countries low maternal vitamin A stores combined with increased demands of pregnancy and lactation may lead to its deficiency in breastfed infants. This study evaluates the effects of maternal supplementation with a high dose of vitamin A on the serum retinol levels of exclusively breastfed infants, and their morbidity in the first six months of life. Setting: Hospital based. Study design: Randomised controlled trial. Subjects and methods: Mothers of the test group (n=150) were orally supplemented with a single dose of retinol (209 µmol) soon after delivery and were advised exclusive breastfeeding for six months. Before supplementation retinol levels were estimated in the mothers' and newborns' blood, and in colostrum. On follow up, breastmilk and infants' serum retinol contents were assessed monthly for six months. Retinol level <0.7 µmol/l indicated vitamin A deficiency. Morbidity patterns like vitamin A deficiency, diarrhoea, febrile illnesses, acute respiratory infection, measles, and ear infection were also studied and compared between the two groups. Results: Presupplement mean maternal serum retinol levels were 0.98 and 0.92 µmol/l and mean breastmilk levels were 3.85 and 3.92 µmol/l in the test and control groups respectively (p>0.05). Mean cord blood retinol levels were also comparable (0.68 v 0.64 µmol/l). After supplementation, the test group showed a rise in mean breastmilk retinol content (12.08 v 2.96 µmol/l) which remained significantly higher for four months. The infants' mean serum retinol level, initially 322.06% of the baseline value, was significantly higher for five months. In the control group, significant numbers of mothers and infants showed deficient breastmilk and serum retinol throughout the follow up (p<0.01). Decreased incidence and duration of various diseases were also found in the test group suggesting lesser morbidity. Conclusion: Maternal supplementation with single megadose vitamin A is an effective strategy for vitamin A prophylaxis of exclusively breastfed infants of 0-6 months.