Background: A double-blind, placebo-controlled randomized trial was designed to investigate the effects of recombinant human granulocyte colony-stimulating factor (rhG-CSF) on the prognosis of preterm infants with early onset neonatal sepsis (EONS). Methods: Fifty preterm infants were selected from the neonatal intensive care unit (NICU) of Qaem Hospital, Mashhad, Iran in 2011. They were randomized to rhG-CSF (intervention group, n = 25) or identical placebo (control group, n = 25) for 3 days. The following blood parameters were measured: White blood cells count (WBC), absolute neutrophil count (ANC), serum level of highsensitivity C-reactive protein (hs-CRP), and the ratio of immature to total neutrophils. In addition, the mortality rate, adverse effects, and duration of hospital stay were evaluated as clinical parameters. Results: At baseline, both groups were not significantly different (P > 0.05) except for the hs-CRP level (P = 0.024) and hypoglycemia (P = 0.001). Compared with the controls, significant improvements were only observed in WBC (P = 0.001) and ANC (P = 0.010) of the intervention group. The mean difference in the WBC, ANC, hs-CRP level, and the ratio of immature to total neutrophils between the baseline and 3-day post-treatment values was higher in the controls than the intervention group. More than 90% of the patients exposed to either rhG-CSF or placebo hospitalized for over 72 hours and no significant difference was found between them (P = 0.946). In each group, a decease was recorded (4.0%) during the hospitalization. Conclusions: The rhG-CSF administration could effectively improve WBC and ANC. No significant changes were observed in mortality rate, adverse effects, and hospital stay after the treatment.