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This study was undertaken to clarify the relation between transient increases in the numbers of leukocytes and granulocyte colony-stimulating factor (G-CSF) plasma concentrations as well as the degree of apoptosis, as determined by binding of annexin V by these cells in patients with severe sepsis and septic shock. Over a 6-month period, annexin V binding by leukocytes was determined daily using flow cytometry and FITC-labeled annexin V in 33 postoperative patients with severe sepsis or septic shock during their intensive care unit stay. The percentage of annexin V binding neutrophils, monocytes, and lymphocytes was significantly lower, and G-CSF plasma concentrations were higher in patients than in controls on most days. Nine, 19, and 18 peaks in neutrophil, monocyte, and lymphocyte counts (increase of >/=30% within 2 days, followed by a decrease of >/=30% within 2 days) occurred in 6, 11, and 16 patients. During leukocyte peaks, the absolute numbers of annexin V binding neutrophils, monocytes, and lymphocytes paralleled those of neutrophil, monocyte, and lymphocyte numbers. However, the percentage of annexin V binding neutrophils, monocytes, and lymphocytes did not differ significantly from one day to another. Increased G-CSF serum concentrations preceded neutrophil and monocyte peaks. In conclusion, apoptosis of leukocytes is lowered during severe sepsis and septic shock in critically ill patients. Moreover, the degree of apoptosis does not increase during transient leukocytosis. G-CSF might contribute to the low degree of apoptosis of neutrophils and monocytes in those patients.