The two most frequently examined biomarkers in the diagnosis of dementia are cerebrospinal fluid (CSF) tau and β-amyloid_1–42 (Aβ_1–42). An assay for tau phosphorylated at threonine 181 (phospho-tau) has recently been developed. We studied these three markers in patients with possible Alzheimer’s disease (AD; n = 23), probable AD (n = 50), AD with relevant cerebrovascular disease (AD with CVD; n = 14), possible vascular dementia (VaD; n = 39), probable VaD (n = 36), cognitively impaired (n = 13) and 27 neurologically healthy controls. Compared with the controls, tau levels were significantly increased in possible AD, probable AD, AD with CVD and probable VaD. Aβ_1–42 was decreased in all dementia groups compared with the controls. In contrast, phospho-tau levels were increased only in probable AD compared with the controls. From the results of the present study, it is concluded that neither measurement of phospho-tau, tau nor Aβ_1–42 in CSF can discriminate entirely between dementia and cognitively non-disturbed controls or between dementia of different aetiologies in the clinical diagnostic procedure.