Replace the term "Hemorrhagic Disease of the Newborn" (HDN) by "Vitamin K Deficiency Bleeding" (VKDB), as neonatal bleeding is often not due to VK-deficiency and VKDB may occur after the 4-week neonatal period. BACKGROUND VKDB is bleeding due to inadequate activity of VK-dependent coagulation factors (II, VII, IX, X), correctable by VK replacement. METHODS In a bleeding infant a prolonged PT together with a normal fibrinogen level and platelet count is almost diagnostic of VKDB; rapid correction of the PT and/or cessation of bleeding after VK administration are confirmative. WARNING SIGNS: The incidence of intracranial VKDB can be reduced by early recognition of the signs of predisposing conditions (prolonged jaundice, failure to thrive) and by prompt investigation of "warning bleeds". METHODS VKDB can be classified by age of onset into early (<24 h), classical (days 1-7) and late (>1 week <6 months), and by etiology into idiopathic and secondary. In secondary VKDB, in addition to breast feeding, other predisposing factors are apparent, such as poor intake or absorption of VK. VK-PROPHYLAXIS: RESULTS Oral and intramuscular VK (one dose of 1 mg) protect equally well against classical VKDB but intramuscular VK is more effective in preventing late VKDB. The efficacy of oral prophylaxis is increased with a triple rather than single dose and by using doses of 2 mg vitamin K rather than 1 mg. Protection from oral doses repeated daily or weekly may be as high as from i.m. VK. VK-PROPHYLAXIS: METHODS VK is involved in carboxylation of both the coagulation proteins and a variety of other proteins. Because of potential risks associated with extremely high levels of VK and the possibility of injection injury, intramuscular VK has been questioned as the routine prophylaxis of choice. Protection against bleeding should be achievable with lower peak VK levels by using repeated (daily or weekly) small oral doses rather than by using one i.m. dose. BREAST FEEDING MOTHERS TAKING COUMARINS: Breast feeding should not be denied. Supervision by pediatrician is prudent. Weekly oral supplement of 1 mg VK to the infant and occasional monitoring of PT are advisable. CONCLUSIONS VKDB as defined is a rare but serious bleeding disorder (high incidence of intracranial bleeding) which can be prevented by either one i.m. or multiple oral VK doses.