Apolipoprotein E (apoE) has been suggested to play a role in regenerative processes in the brain after trauma, and also in the pathogenesis of Alzheimer's disease (AD). We examined cerebrospinal fluid (CSF) apoE in a material consisting of 23 patients with early-onset AD (EAD), 31 with late-onset AD (LAD), 16 with frontal-lobe dementia (FLD), 25 with vascular dementia (VAD) and 25 controls. CSF-apoE was decreased in all of EAD (1.8 +/- 1.1 mg/l; p < 0.0005), LAD (2.5 +/- 0.9 mg/l; p < 0.0005), VAD (2.3 +/- 1.4 mg/l; P < 0.0005) and FLD (3.0 +/- 1.3 mg/l; p < 0.05) compared to the control group (5.7 +/- 4.0 mg/l). Since apoE4 has been found to bind to beta/A4-amyloid, and AD patients homozygous for apoE4 to have higher number of senile plaques than apoE3 homozygotes, we also examined the relation between CSF-apoE and apoE alleles. However, CSF-apoE did not significantly differ between patients with different apoE isoforms. Our findings support that apoE is involved in the pathogenesis of dementia disorders, both degenerative and vascular, but the CSF-apoE level is not influenced by the apoE isoforms. CSF-apoE may be used as an unspecific marker for neurodegenerative disorders, but not in purpose of differential diagnostics between different dementia disorders.