Apolipoprotein E (ApoE) is associated with Alzheimer's disease (AD) neurofibrillary tangles and beta-amyloid protein in senile plaques. There are three common alleles of ApoE, designated epsilon 2, epsilon 3 and epsilon 4. We studied Finnish patients with neurodegenerative disorders: AD, vascular dementia (VAD), Parkinson's disease (PD), PD+dementia (PDD), Lewy body variant of AD (LB), frontal dementia (FD), and Down's syndrome (DS), as well as control individuals (C). The ApoE genotypes and corresponding allele frequencies of 188 patients and 60 controls were determined by digestion of ApoE polymerase chain reaction products with the restriction enzyme Hha I. The ApoE epsilon 4 allele frequency was 0.17 for C, 0.44 for AD, 0.35 for VAD, 0.10 for PD, 0.38 for PDD, 0.28 for LB, 0.39 for FD, and 0.17 for DS. We found significant differences in genotype frequency between AD/C, AD/PD and AD/DS. Our results suggest that, beside AD, an increased frequency of epsilon 4 may also be involved in other dementing neurological disorders.