The prevalence of vitamin K deficiency in the newborns delivered at Siriraj Hospital was studied. The prolongation of one stage prothrombin time and the presence of PIVKA-II (non carboxylated prothrombin antigen) in cord blood were interpreted as the secondary change from vitamin K deficiency state. The most reliable method to diagnose vitamin K deficiency is the detection of vitamin K level in plasma which is not yet available in Thailand. Although the prevalence of vitamin K deficiency in the newborns from our data is not high, only 0.6%, it is shown that some of the apparently normal newborn infants may have bleeding problem from vitamin K deficiency in both newborn and early infancy periods. So, the correction of this deficiency by administration of vitamin K to all newborns is appropriate and reasonable decision.