The administration of granulocyte-macrophage colony-stimulating factor (GM-CSF) to patients with severe active infections has been questioned because activation of neutrophils may cause tissue injury. To identify the effect of GM-CSF administration on severe sepsis, we examined the survival rate and pathologic changes in vital organs using the rat lethal sepsis model. Rats received 20 micrograms of recombinant murine GM-CSF (rmGM-CSF) 3 hours after the onset of peritonitis induced by cecal ligation and puncture. After 48 hours, the survival rate did not improve, and earlier deaths than in the control group were observed. In addition, the inhibition of early leuko-sequestration in the peritoneal cavity was seen in animals treated with GM-CSF. These results suggested that the administration of rmGM-CSF after the onset of sepsis was not beneficial; thus, we concluded that care should be taken in the clinical use of GM-CSF in severe infection.