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Vitamin K has been detected more than eighty years ago, considered as a very important agent in hemostasis. There are several natural forms of vitamin K: phylloquinone (vitamin K1) from plants, menaquinones (vitamin K2) are produced by bacterial intestinal flora, while menadione (vitamin K3) is considered as the intestinal catabolic product of oral phylloquinone which is considered as the circulatory precursor of menaquinone-4. Besides in the coagulation, vitamin K has an important role in many other physiological processes by making some important proteins active. Vitamin K does not influence the production of the primary gene product, while its action is very important on posttranslational proteins. Besides that vitamin K increases exchange between sister chromatids, and it has oxidative and antioxidative effects. Hemorrhagic disease of newborn (HDN) or vitamin K deficiency bleeding is acquired disease affecting infants suffering vitamin K deficiency. According to the onset of symptoms, there are three different forms of HDN: early (onset of bleeding within 24 hours after birth), classic (onset of bleeding between the 2nd and the 7th day of life), and late (onset of bleeding from the 8 days through the entire infancy). Intracranial hemorrhage is the most frequent manifestation of the early and the late vitamin K deficiency bleeding. That was the reason why more than fifty years ago, (in 1961) American Academy of Pediatrics issued recommendations on prevention of the HDN. Despite many controversies connected with the prophylactic use of vitamin K, still, intramuscular vitamin K administration is considered the most efficient and safest method for the prevention of late HDN, while vitamin K is used in the treatment of bleeding due to vitamin K deficiency in infancy. The most recent Croatian recommendations for the prevention of HDN are in concordance with many actual international recommendations. [ABSTRACT FROM AUTHOR]