Cerebrospinal fluid (CSF) levels of neuron-specific enolase (NSE) were measured in 22 patients with probable Alzheimer's disease and in 35 patients with multi-infarct dementia, and in 15 controls. CSF NSE in patients with Alzheimer's disease did not differ from those in controls. In patients with multi-infarct dementia without recent vascular events CSF NSE was lower than in controls or in Alzheimer patients. This finding is in accord with the prevailing opinion that vascular dementia is caused by multiple infarcts and not by continuous neuronal ischaemia.