The objective of this study was to investigate the effect of treatment with recombinant human granulocyte-colony stimulating factor (rhG-CSF) on the neutrophil count and function of preterm neonates with documented sepsis. For this purpose 62 preterm neonates with proven sepsis and 19 healthy preterm ones were studied. Of the 62 patients, 27 septic neonates had an absolute neutrophil count (ANC) > 5000/mm3 (group A) and were scheduled not to receive rhG-CSF and 35/62 had an ANC < 5000/mm3 (n=35) and were randomly assigned either to receive rhG-CSF (group B) or not to receive it (group C). rhG-CSF (10 microg/kg) was administered for 3 consecutive days (0, 1, 2). The ANC, plasma levels of G-CSF (ELISA), neutrophil respiratory burst activity (NRBA) and neutrophil expression of CD11a, CD11b and CD11c (flow cytometry) were measured in all septic neonates on days 0 (onset of sepsis), 1, 3 and 5 and in the healthy neonates once within the first 2 days of life. We found that on day 0, G-CSF levels of all groups of septic neonates were significantly higher than those of the healthy ones. The highest levels were observed in group A. NRBA was diminished only in groups B and C and the expression of CD11a and CD11c was reduced in all groups of septic neonates. Administration of rhG-CSF resulted in a rapid and significant increase in ANC, NRBA and CD11a, CD11b and CD11c expression that persisted throughout the follow up. CONCLUSION; The administration of granulocyte colony stimulating factor to septic neonates significantly increases the absolute granulocyte count and enhances the neutrophil respiratory burst and beta2 integrin expression.