Eosinophilia is a common finding in premature babies during the neonatal period (75%). It has a variable correlation with several factors that may influence it: gestational age, birth weight, time required to regain birthweight, neonatal stress, caesarean birth, formula feeding, endotracheal intubation, positive pressure ventilation, parenteral nutrition, umbilical catheterization, blood transfusions, antibiotics, phototherapy, sex, neutropenia and sepsis. The possible mechanisms are discussed: resolution of a stress, anabolic state, reaction to antigens, normal granulopoietic maturation, transplacental transfer of a factor stimulating the eosinophil synthesis.