The nervous system specific proteins glial fibrillary acidic protein (GFAp), S-100 and myelin basic protein (MBP) were determined in 535 human cerebrospinal fluid (CSF) samples. The level of all three proteins was increased in CSF of patients with nonselective destructive central nervous tissue disease such as encephalitis, cerebrovascular disease or tumoural compression. The increases in GFAp were more constant, making it a better marker of CNS pathology. Increases in MBP in CSF of patients with acute demyelinating disease were confirmed. S-100 did not seem to give more information as GFAp. Isolated increases of GFAp could be demonstrated in patients with dementia (Alzheimer type or multi-infarct dementia) or syringomyelia. Since CNS of these patients is very rich in fibrillary astrocytes, containing large amounts of GFAp, it is suggested that GFAp is to be considered as a specific marker of fibrillary gliosis in CSF and can be used as a diagnostic tool in dementia and syringomyelia.