Vitamin K has regained paediatric interest due to a recurrence of bleeding caused by deficiency of the vitamin in newborns and young infants. Increasing awareness of these clinical problems, the development of new methods for the detection of vitamin K deficiency and the direct measurement of vitamin K in tissues have stimulated research. Much new data obtained from these studies has proved helpful to the understanding of vitamin K deficiency in infancy. For example low concentrations of vitamin K have been found in fetal and neonatal livers. The implications of these findings with respect to manifest vitamin K deficiency and to new methods for detection of subclinical vitamin K deficiency are discussed. Breast-feeding is a major risk factor for classical haemorrhagic disease of the newborn and for late onset bleeding due to vitamin K deficiency in young infants. The interdependencies between breast-feeding and vitamin K deficiency are discussed on the basis of new data obtained from direct measurement of vitamin K in maternal milk. The review further focuses on pathophysiological concepts of bleeding due to vitamin K deficiency in infancy and current concepts of vitamin K prophylaxis.