Granulocyte-colony stimulating factor (G-CSF) and granulocyte macrophage colony-stimulating factor (GM-CSF) are being administered to patients with neutropenia. However, little is known about the endogenous levels of both factors in these patients. We measured the endogenous G-CSF levels in patients with chemotherapy-induced neutropenia (n=15, study group A), in patients who had not received chemotherapy with neutropenia caused by a number of primary diseases (n=14, study group B) and in healthy volunteers (n=15, control group). Both the study groups and the control group did not show any clinical or laboratory findings of infection. The G-CSF levels were elevated in patients following chemotherapy and in patients who had neutropenia without chemotherapy, but the mean G-CSF levels in patients with chemotherapy-induced neutropenia were significantly higher than in patients with primary diseases. The levels of endogenous G-CSF were also higher in both neutropenic groups, compared to the control group. In conclusion, endogenous G-CSF levels in chemotherapy-induced neutropenia were significantly higher than non-chemotherapy related neutropenia and controls. This may be explained as G-CSF synthesizing bone marrow stromal cells may be more affected in primary disease related neutropenia than in chemotherapy induced neutropenia.