The concentration of glial fibrillary acidic protein (GFAP) in lumbar cerebrospinal fluid (CSF) was measured in 12 patients with normal pressure hydrocephalus (NPH) 11 patients with primary degenerative dementia (PDD), 8 patients with various other neurological diseases, and 18 patients without signs of organic nervous disease (controls). Mean CSF GFAP concentration was significantly higher in NPH patients: 96 +/- 23 ng/ml (SEM) when compared with PDD patients: 8.2 +/- 1.9 ng/ml (P less than 0.01), or with controls: 4.3 +/- 0.7 ng/ml (P less than 0.01). Only 2 NPH patients had a GFAP concentration within the range of the control group (2-14 ng GFAP/ml CSF). No significant differences were found between the PDD patients and the control group, or between the group of patients with other neurological diseases and the control group. In addition, a rostro-caudal gradient of GFAP in CSF could be demonstrated. In 6 NPH and 2 PDD patients both ventricular and lumbar CSF samples were investigated. In all cases the ventricular GFAP concentration was higher than the lumbar concentration. The difference was statistically significant (P less than 0.01). Our results suggest that determination of CSF GFAP concentration might be of diagnostic value in discrimination between NPH patients and patients with enlarged ventricles associated with degenerative brain disease.