To gain insight of the underlying mechanisms of astroglial response to Alzheimer’s disease (AD), the level of glial fibrillary acidic protein (GFAP) in cerebrospinal fluid (CSF) from controls and AD subjects were immunochemically determined, and the correlation between that level and dementia severity of AD patients was evaluated. Means and SD of CSF levels of GFAP for the young control group (from 1 to 25 years, mean±SD 14.2±5.0, n= 13) adult control (from 26 to 55 years, 41.6±10.1, n= 9) and senescent control (older than 56 years, 65.4±8.0, n= 8) were 2.96±1.04, 2.80±1.46 and 3.99±1.55 ng/ml, respectively, and the CSF level of GFAP was not dependent on age (ANOVA, p= 0.17). While that of the AD patient group (n= 27, 70.8±8.0 years) was 8.96±7.80 ng/ml, significantly higher than that of both the all-control (3.19±1.39 ng/ml, t test, p< 0.001) and age-matched senescent (3.99±1.55 ng/ml, t test, p< 0.005) control groups. The receiver-operator characteristic (ROC) curve revealed that the GFAP concentration at 5 ng/ml in CSF could serve as a cutoff value. The CSF level of GFAP in the moderately to severely demented patients (MMSE≤ 17, 13.2±9.10 ng/ml, n= 9) was approximately by two-fold higher than that of the mildly to moderately demented patients (MMSE≧ 18, 6.85±5.76 ng/ml, n= 18; ANOVA, p< 0.05). These findings together with our previous report on an increase in the CSF level of apolipoprotein E suggest that degeneration and stimulation of astrocytes takes place concurrently in the AD brain.