Tau protein levels in cerebrospinal fluid (CSF-tau) were determined in 29 patients with old cerebrovascular disease (CVD, 21 demented and eight non-demented), 69 patients with Alzheimer's disease (AD) and 17 age-matched normal controls. The CSF-tau level in the vascular dementia (VD) group (24.0 +/- 17.0 pg/ml) was significantly lower (P < 0.0001) than that in the AD group (90.0 +/- 45.3 pg/ml), but not significantly different from that in the non-demented patients with CVD (18.1 +/- 10.2 pg/ml) or controls (20.3 +/- 13.0 pg/ml). Among the VD patients, 1/21 exceeded a cut-off value (mean +/- 2 SD of controls), whereas 8/69 of the AD patients had CSF-tau levels below this value. These findings suggest that VD constitutes a group of dementias that can be separated from AD by normal CSF-tau levels. CSF-tau determinations in combination with other clinical findings may provide another diagnostic aid in the differential diagnosis between VD and AD.