For regions where vitamin A deficiency is common, high dose vitamin A supplementation of lactating mothers is currently recommended, but the effects of this intervention have not been carefully evaluated. We conducted a randomized, double-blind trial in which 153 Indonesian mothers 1-3 wk postpartum received either a capsule containing 312 mumol of vitamin A as retinyl palmitate or a placebo. Mothers' serum retinol concentrations in the vitamin A group tended to be lower than in the placebo group at baseline but higher at 3 mo postpartum (1.39 vs. 1.24 mumol/L, P = 0.03) and 6 mo postpartum (1.23 vs. 1.08 mumol/L, P < 0.01). The milk retinol concentrations of the vitamin A group were higher than those of the placebo group by 0.48 to 1.18 mumol/L at 1-8 mo postpartum (P < 0.05). Among the infants at 6 mo of age, the prevalences of low serum retinol concentration (< 0.52 mumol/L) were 36 and 15% in the placebo and vitamin A groups, respectively (P < 0.005), and the prevalences of low vitamin A stores (assessed by relative dose response) were 23 and 10%, respectively (P < 0.03). High dose vitamin A supplementation of lactating mothers is an efficacious way to improve the vitamin A status of both mother and breast-fed infant.