We studied whether the administration of vitamin K to mothers could increase the concentration of vitamin K in breast milk and prevent idiopathic vitamin K deficient bleeding in breast-feeding infants. Sixty puerperal women were divided into three groups, the control group, Menaquinone-4 (MK-4) administered group and vitamin K1 administered group. We measured the concentrations of vitamin K1, MK-4 and MK-7 in maternal plasma and breast milk on the fourth day after delivery. In the MK-4 group, the concentrations of MK-4(2.13 ng/ml in plasma, 49.3 ng/ml in milk) were significantly higher than in the control group (0.28 ng/ml, 1.51 ng/ml). In the vitamin K1 group, the concentrations of vitamin K1 (49.0 ng/ml in plasma, 71.6 ng/ml in milk) were significantly higher than in the control group (1.17 ng/ml, 2.41 ng/ml). The concentration rates (milk/plasma ratio) of vitamin K1, MK-4 and MK-7 were 2.52, 5.43 and 0.52 in the control group, 1.60, 40.2 and 0.67 in the MK-4 group and 1.65, 10.8 and 0.71 in the vitamin K1 group, respectively. The concentration rate of MK-4 was higher than that of vitamin K1 and was increased by MK-4 administration. After delivery, the daily concentration of MK-4 in milk was increased from 1.69 ng/ml on the first day to 49.3 ng/ml on the fourth day in the MK-4 group. These results indicate that MK-4 is accumulated and concentrated into breast milk, and continuous MK-4 administration can increase the concentration of vitamin K in milk, preventing idiopathic vitamin K deficient bleeding in infants.