Neonatal vitamin K deficiency causes early and classic HDN (hemorrhagic diseases of the newborn). There have been some suggestions as to how to prevent from having vitamin K deficiency in babies. However, early HDN could not be prevented by vitamin K administration after birth even as soon as possible. To prevent early HDN, we have attempted to administer vitamin K_2 syrup to 15 mothers before delivery, and measured vitamin K_2 in maternal and umbilical cord plasma at birth and breast milk 5 days after delivery. The mean K_2 values in maternal and umbilical cord plasma and in breast milk were significantly high as compared with those of 15 cases of control. Therefore, oral administration of vitamin K_2 syrup to mother before delivery is more preferable than intramuscular injection to neonate, which involves a certain degree of danger. This preventive method is safe, and causes early and continuous transportation of vitamin K_2 from mother to neonate.