Introduction: Newborns exclusively breast-fed (EBF) may present low levels of glycemia in the first hours of life and their monitoring is very important. Severe hypoglycemias are associated with neurological sequels. Objectives: To determine the influence of breastfeeding exclusive or not over glycemia levels in the newborn during the first 24 hours of life. Design: Cuasiexperimental kind of study. Setting: Immediate care and child care services, Instituto Nacional Materno Perinatal, Lima, Peru, a teaching hospital. Participants: Breast-fed newborns. Interventions: Ninety newborns were distributed in 3 groups according to the kind of feeding: a) 30 EBF; b) 30 breastfeeding plus maternized milk; and, c) 30 breastfeeding plus glucose solution. Newborns admited fulfilled criteria of inclusion and previous consent of the mother a sample of blood was taken from the mother and the newborn to d...