Bacterial sepsis is a major cause of morbidity and mortality in neonates. Granulocyte colony stimulating factor (G-CSF) is a physiologic regulator of neutrophil production and function. The aim of this study was to deter¬mine if serum granulocyte colony stimulating factor level could be of value in diagnosing neonatal sepsis. We measured serum G-CSF levels by enzyme linked immunoassay in 70 newborn infants of varying gestational ages and postnatal ages. We classi¬fied the infants into 3 groups: group I (18 infants) with positive bacterial blood culture result; group II (25 in¬fants) with negative blood culture re¬sult but evidence of clinical sepsis, and group 111 (27 infants) with nega¬tive blood culture and no evidence of infection (control group). The mean G-CSF level was 2312 ± 396 pg/ml ingroup I, 2173 ±415 pg/ml in group II, and 166 ± 19 pg/ml in group III. The mean G-CSF levels of group I and 11 were significantly higher (P