Because of the many functions of vitamin A in human physiology, deficiency or excess of the vitamin in lactating women or their infants can adversely affect their health. Infants are born with low body stores of vitamin A, and rely on vitamin A in milk to meet their needs. The vitamin A content of milk is related to maternal vitamin A status and maternal dietary vitamin A intake during lactation. Low-income lactating women in non-industrialized countries have lower milk vitamin A concentrations than lactating women in industrialized countries. Supplementation of lactating women in non-industrialized countries with vitamin A or beta-carotene has resulted in increased milk vitamin A concentrations. However, the optimal timing and dose for sustaining adequate levels of vitamin A in milk throughout the lactation period has not been determined. Further research is needed to understand factors affecting the transfer of vitamin A to milk, and to evaluate various strategies for improving the vitamin A status of mothers and infants.