Cerebrospinal fluid (CSF) levels of the anxiogenic neuropeptide diazepam binding inhibitor (DBI) were determined by radioimmunoassay in 281 patients who underwent evaluation for neurological problems. Serial dilution curves and reverse-phase high pressure liquid chromatography showed that the immunoreactive material in CSF behaved just as authentic DBI extracted from human brain. Furthermore in the assay there was no evidence of interference from CSF samples deprived of DBI by immunoaffinity. In 82 patients with no evidence of major lesions in the central nervous system, who acted as controls, the CSF DBI content was shown to be age- and sex-related. No correlation was observed with the CSF protein concentration. In patients with different types of dementia, the levels of CSF DBI were significantly increased in a group with normal pressure hydrocephalus. No significant differences were found between Alzheimer's disease, multi-infarct dementia, or dementia with Parkinson's disease and controls. In non-demented patients with Parkinson's disease the levels of DBI were increased in a subgroup with depressive disturbances whereas no differences was observed in the non-depressed cases. The content of DBI was markedly reduced in 5 cases with olivopontocerebellar atrophy and in 4 with spinocerebellar ataxia. In all the other disorders studied the levels of DBI were similar to or slightly lower (multiple sclerosis) than those of the controls. The origin of DBI in cerebrospinal fluid is uncertain; a number of various possibilities are discussed concerning the proposed role of DBI as modulator of brain GABAergic transmission.