At the early stage of aseptic meningitis, there is a transient increase in neutrophil counts in the cerebrospinal fluid. Some factors in the cerebrospinal fluid might induce migration of neutrophils into the cerebrospinal fluid. Granulocyte colony-stimulating factor (G-CSF) plays an important role, not only as a hemopoietic factor but also as a regulating factor for a biologic defense system by neutrophils in the foci of infection. To analyze the role of G-CSF on accumulating neutrophils in the cerebrospinal fluid, we have measured G-CSF levels in the cerebrospinal fluid of children with aseptic meningitis, paying particular attention to the phasal transition. Within the first 2 d from the onset, G-CSF levels in the cerebrospinal fluid were 223 ± 97 ng/L, significantly higher than those of the patients without meningitis (p< 0.01). Beyond the second day after the onset, the G-CSF levels rapidly decreased to below the detectable level, even though the patients manifested meningeal signs and symptoms. There was a direct relationship between G-CSF levels and neutrophil counts in the cerebrospinal fluid (r= 0.763,p< 0.01). During the first 2 d after the onset, the G-CSF level in the cerebrospinal fluid in each case was remarkably higher than that in the serum. This finding suggests that the G-CSF in the cerebrospinal fluid was produced in the spinal cavity. From our results, the transient elevation of G-CSF levels might lead to the transient increase in neutrophil counts in the cerebrospinal fluid by recruiting them from the peripheral blood at the initial stage of aseptic meningitis.