To identify the therapeutic efficacy of granulocyte colony stimulating factor (G-CSF) in severe sepsis, we examined its effect on the mortality and pathological changes in vital organs using the rat lethal sepsis model. Rats were given 15 micrograms of recombinant human (rh)G-CSF after the onset of peritonitis brought about by cecal ligation and puncture. The mortality rate after 72 h was significantly decreased by administration of 15 micrograms of rhG-CSF (p < 0.001). In addition, the administration of rhG-CSF induced an improvement in liver and renal functions. It also produced marked pathological improvement in the lungs. These results strongly indicated that administration of rhG-CSF, even after the onset of sepsis, was effective in decreasing the mortality from peritonitis-induced multiple organ failure, and this finding was clearly useful in the clinical treatment of such sepsis-induced critical illness.