In recent studies, patients diagnosed with Alzheimer's disease (AD) showed significantly elevated CSF levels of tau protein. Tau protein was therefore regarded as a putative molecular marker for AD. Since early diagnosis of AD is warranted for appropriate therapeutic intervention, investigation of total tau protein levels in patients with Aging Associated Cognitive Decline (AACD) and AD are reasonable. In our study the CSF concentrations of total tau protein were measured by ELISA in 132 patients with AD, 29 patients with AACD and 24 healthy controls. CSF concentrations were compared between the subgroups of mild, moderate and severe AD, AACD and the control group and were correlated with age and severeness of the illness. The concentration of total tau protein was increased significantly in patients with severe and moderate AD compared to all other groups. Within the group of AD patients, total tau protein correlated significantly with the severity of the dementia but not with age. Although the range of the measured tau protein concentrations is wide and overlapping between the diagnostic groups our data indicate that from a clinical point of view significantly increased tau protein levels confirm the clinical diagnosis of AD while normal values do not exclude it.