Neonates on exclusive breast feeding that do not receive vitamin K at birth are at higher risk hemorrhagic disease of the newborn.To compare the effect of oral or intramuscular administration of vitamin K1 (VK1), on clotting factors II, VII, IX, X and PIVKA II, in children until the 60 days of age with exclusive breast feeding or mixed feeding.Forty healthy full term infants, distributed in two groups, A: 20 with mixed feeding (formula-feeding and breast-feeding) and B: 20 with exclusive breast feeding, were studied. Nine infants of each group received 1 mg of VK1 intramuscularly and eleven 2 mg VK orally 5 ml of cord blood was collected initially from each infant. Venous blood samples were taken on 15, 30 and 60 days of age.All factors increased in a progressive form reaching levels over 50% at 60 days of age, in both groups. PIVKA II decreased significantly during the study period (p < 0.01). Factor II increased more in children with mixed feeding that received intramuscular vitamin K, than in the rest of study groups. No other differences between groups were observed. No infant had an abnormal bleeding during the study period.Oral administration of vitamin K is as effective as the intramuscular route in the prevention of the hemorrhagic disease of the newborn.