Breast-feeding is the sole source of vitamin K for most of the world's children and breast-fed infants are at risk for vitamin K-responsive hemorrhagic disease of the newborn (HDNB). Recent advances in high performance liquid chromatography methodology have made possible the first quantitative studies of vitamin K in human milk. Although much progress has been made, much remains to be done. Innovative improvements in methodology are needed, as detection of nanogram quantities of vitamin K in milk is at the limit of current methodology. Additional studies are needed over the lactation period. A better understanding of colostrum is needed with regard to other nutrients as well as vitamin K. Vitamin K in the milk of mothers who gave birth prematurely has not been measured. The significance of menaquinones as a vitamin K source to the infant is undetermined. The mechanism regulating vitamin K secretion into milk has not been investigated. The localization of vitamin K in milk is undetermined as is the relationship of vitamin K to other milk lipids. The effects of fat-soluble vitamins in the diet on vitamin K concentrations in milk is unknown. The pharmacokinetics of vitamin K supplementation of mothers is particularly important in cultures where vitamin K is not routinely administered at birth. Finally, most critical at this point is our ignorance about the relationship of the maternal vitamin K status to the vitamin K status of the infant. As breast milk is the sole source of vitamin K for most of the world's infants, HDNB remains a very real threat to the health of infants and warrants concentrated study.