Protein tau concentration was determined by an enzyme linked immunoassay (ELISA) in cerebrospinal fluid (CSF) of 22 patients with clinically diagnosed Alzheimer's disease (AD), in three patients with dementia caused by other neurological disorders (OND) and in 19 cognitively healthy controls. A significantly elevated protein tau concentration was found in AD patients as compared with controls, even in 11 patients with very mild dementia. There was no correlation between protein tau and the severity of cognitive impairment as assessed with the Mini Mental State Examination (MMSE). Tau concentrations were not or only slightly elevated in OND. We conclude that protein tau in CSF might be helpful as a biological marker for the early diagnosis of AD.