Extremely immature preterm infants rarely present with a leukocytosis exceeding 30,000/microL. The pathogenetic sequence leading to leukemoid reactions in non-malignant diseases remains to be elucidated. Potential triggers for leukemoid reactions in premature infants include prenatal corticosteroids, chorioamnionitis and funisitis or systemic infection. In the two-year period from 2006 to 2007 all infants with a gestational age of less than 26 weeks were screened for leukocytosis. Among our cases, one preterm infant presented with a leukocyte count of 229,300/microL at the age of 48 hours, lasting throughout the first three weeks of life. Impairment of microcirculation and resulting organ dysfunction were not observed. Thus, invasive therapeutic procedures, which are routinely initiated in hyperleukocytosis in accompanying malignant diseases, may not have the same significance in extremely immature preterm infants and should be executed in these patients on an individual basis and with extreme caution.