The granulocyte colony-stimulating factor (G-CSF) regulates neutrophil differentiation and function. Serum levels of G-CSF increase during acute infectious processes. The levels of G-CSF were measured in 59 surgical intensive care unit (ICU) patients. In general, G-CSF was only elevated during the first 2 days after admission to the ICU. Higher G-CSF levels were more frequently observed in patients without infectious complications and in patients who survived. Later on, G-CSF levels were below 100 pg/ml in almost all patients studied. The highest G-CSF level (20,000 pg/ml) was observed in one patient with septic shock 36 h after leukopenia. The patient recovered from septic shock and multiple organ failure and was discharged. It is proposed that surgical ICU patients with low or undetectable G-CSF serum levels may benefit from exogenous G-CSF substitution protocols.