To clarify the patho-physiologic role of granulocyte colony-stimulating factor (G-CSF) in chronic myeloid leukemia (CML), we determined the serum levels of G-CSF in various stages of CML using a very sensitive method: chemiluminescence enzyme immunoassay (CLEIA). This method makes it possible to estimate very low levels of serum G-CSF. In the present study, serum samples from 25 patients in chronic phase and 16 in blastic crisis, as well as samples from 33 healthy volunteers were investigated. The serum G-CSF levels in chronic phase of CML (2.95 +/- 3.91 pg/ml) were significantly lower than those in normal controls (15.92 +/- 6.53 pg/ml) and in blastic crisis of CML (15.52 +/- 17.65 pg/ml) within a range of very low levels (p < 0.001, p < 0.02). Moreover, a reverse correlation between blood neutrophil counts and serum G-CSF levels were clearly demonstrated for CML including blastic crisis (r = 0.405, p < 0.02). Interestingly, a sequential parallel relation was observed between serum G-CSF levels and neutrophil alkaline phosphatase (NAP) scores for a patient with CML in chronic phase. Our observations indicate that a negative feedback mechanism exists between peripheral neutrophils and serum G-CSF levels in the chronic phase of CML, and that very low levels of G-CSF in chronic phase of CML might be an important cause for the low NAP scores.