Vitamin K is essential for the formation of at least three proteins involved in blood clotting as well as of other proteins found in plasma, bone, and kidney. Vitamin K deficiency, however, primarily affects the blood clotting process. Vitamin K is provided both from the diet and from endogenous bacterial synthesis, presumably in roughly equal measure. Because intakes of 0.4 micrograms X kg-1 X d-1 (0.89 nmol X kg-1 X d-1) and probably lower intakes maintain normal clotting activities in healthy neomycin-treated adults, rounded daily recommended dietary intakes (RDI) for essentially all reference 76-kg men and 62-kg women are 45 micrograms (100 nmol) and 35 micrograms (78 nmol) phylloquinone, respectively. Newborn infants are at high risk because breast milk contains inadequate concentrations of vitamin K and their intestines are not yet colonized with vitamin K-producing bacteria. Body reserves of vitamin K are small and turnover rapidly. Hence, supplementation of infants with vitamin K is highly advisable. Increments of vitamin K during pregnancy and lactation are also suggested.