The present study concerns an ELISA for glial fibrillary acidic protein (GFAP) in cerebrospinal fluid (CSF). The application of the method in CSF of children has previously been described in this journal. We have now adapted the technique to allow determination of the much higher GFAP concentrations found in normal and pathological CSF of adults. The assay range was extended to 16,000 pg/ml. CSF levels as high as 170,000 pg/ml could be assayed since dilution experiments indicated immunological identity between purified GFAP and GFAP in CSF. In normal controls the concentrations correlated significantly with age (P < 0.001, Spearman rank correlation test). The concentrations were less than 200 pg/ml before 20 years of age and increased to between 500 and 1300 pg/ml at approximately 75 years of age. This increase of CSF GFAP probably reflects formation of astroglial filaments in the CNS and it must be taken into consideration when determining the pathological CSF GFAP level. The method was validated using CSF samples from patients with either astrogliosis or acute tissue destruction in the CNS. Although augmented levels were observed in both groups it is quite clear that patients with acute tissue destruction may display very high CSF GFAP concentrations, whereas levels in patients with astrogliosis were only modestly increased if at all.