Introduction/Objective. Vitamin K deficiency is common in newborn infants and without prophylaxis there is a risk of vitamin K deficiency bleeding (VKDB). The most frequent prophylactic approach is an intramuscular (IM) injection of vitamin K1 immediately after birth. Its efficiency to prevent late VKDB has been recently questioned by several reports. Based on our experience, we discuss the need for additional vitamin K1 supplementation after its IM administration at birth. Methods. We present a retrospective review of 12 infants, 11 with confirmed and one with probable late VKDB despite IM prophylaxis at birth, who were treated in the two largest tertiary care pediatric hospitals in Serbia during the last 15 years. Results. All the patients were exclusively breastfed. In 11 patients, daily weight gain was normal or increased, and one patient had failure to gain weight. Six infants were previously healthy, three infants received antibiotics prior to bleeding, and in two diarrhea and cholestasis, respectively, existed previously. An intracranial bleeding was documented in nine infants, four of whom died. Conclusion. Low content of phytomenadione in human milk could occasionally be attributed to late VKDB despite postnatal IM injection of vitamin K1 in otherwise healthy, exclusively breastfed infants. This might be aggravated by transient disturbance of vitamin K turnover due to antibiotic use, acute diarrhea, or transient cholestasis. We suggest that an additional vitamin K1 supplementation after postnatal IM prophylaxis could be justified in exclusively breastfed infants.