The author confirmed the presence of numerous alterations of the clotting mechanism in the first days of life and wish to stress the marked variability observed in individual infants. Severe alterations may even be present in the first hour of life. The requirements of vitamin K for the newborn are about 25 mg and this dose produces as favorable an effect as larger amounts. Early feeding modifies the alteration but significant improvement is not observed until at about 48 hours. Immaturity has only been shown to produce a decreased capacity to utilize vitamin K. Pathologic conditions in the infant exaggerate such an effect; mild maternal toxemia has no adverse action. Hemorrhages may occur from deficiency of vitamin K. These usually are accompanied by a typical pattern of clotting defect which commonly respond spectacularly to therapy. On the other hand many newborn infants, particularly those of low birth weight, may show extensive hemorrhagic lesions despite the use of vitamin K, and in these there often are multiple complex coagulation defects. Abnormal pro-thrombin times in these infants are not corrected by vitamin K and are often related to low levels of Factor V and f ibrinogen. Alteration of capillary permeability is often present in very small premature infants. Thrombocytopenia may coexist in some. Liver damage appears to play a part in this plasmatic defect and a combination of both disorders has also been observed. Prophylaxis with vitamin K definitely prevents a severe coagulation defect which at times may be entirely responsible for extensive and even fatal bleeding in the newborn. However it is ineffective in the majority of hemorrhagic lesions encountered during the neonatal period.