The neonate is uniquely susceptible to severe and overwhelming bacterial infections. One of the most important deficits in the neonatal host defense system seems to be a quantitative and qualitative deficiency of the myeloid and the phagocytic system. Future optimal therapy of neonatal sepsis may include the use of adjuvant immunologic therapy. Granulocyte colony-stimulating factor (G-CSF) has been shown to induce neutrophilia and to enhance mature effector neutrophil function. To evaluate the role of G-CSF with respect to infection, we examined serum levels of G-CSF in term and preterm neonates, using an enzyme-linked immunosorbent assay method. G-CSF levels in healthy neonates showed peak levels up to 7 hours after birth, followed by an increase in total neutrophil cell (TNC) counts. Both G-CSF levels determined between 4 and 7 hours after birth and peak TNC counts correlated with the gestational age of the neonates. The state of nutrition, maternal treatment with glucocorticoids, maternal infection and hypertension, and the mode of delivery influenced peak G-CSF levels. Neonates with signs of infection between 4 and 7 hours after birth had higher levels of G-CSF than did healthy neonates (1,312 +/- 396 pg/mL v 176 +/- 19 pg/mL). In conclusion, the presented results of serum concentrations of G-CSF in relation to TNC counts and various diseases suggests an important role of G-CSF in the regulation of granulopoiesis during the neonatal period.