Cerebrospinal fluid (CSF) concentrations of tau protein were measured using an enzyme-linked immunosorbent assay which measures both normal and hyperphosphorylated tau. Levels of CSF tau were measured in 17 patients with Alzheimer's disease and 23 patients with frontotemporal dementia, and were compared to age-matched healthy controls. The CSF tau concentration in control subjects was 198+/-49 pg/ml and no relationship was found between age and CSF tau concentrations in these subjects. CSF tau concentrations were significantly raised in patients with both Alzheimer's disease and frontotemporal dementia when compared to healthy controls (802+/-381 pg/ml, P<0.001; 612+/-382 pg/ml, P<0.05, respectively). In neither disease did CSF tau correlate with disease severity as assessed by the Mini Mental State Examination score (MMSE). This study shows that CSF tau is significantly raised in patients with frontotemporal dementia.