The apoE phenotype of 83 patients with probable Alzheimer's disease (AD) and of 164 non-demented controls was determined by isoelectric focusing and Western blotting. The proportion of the epsilon 4 allele was 0.548 in AD and 0.202 in controls (P < 0.0001). The effect was seen in both early-onset and late-onset AD patients. The risk of AD in epsilon 4 homozygotes was 18-fold greater than in individuals without the epsilon 4 allele. ApoE concentrations were measured in serum and cerebrospinal fluid (CSF) from a subgroup of patients with AD (n = 72) and controls (n = 84) by a sandwich enzyme-linked immunosorbent assay. Although serum apoE concentrations were lower in individuals with the epsilon 4 allele than in those without the epsilon 4 allele, CSF apoE concentrations did not vary in different phenotype groups. However, CSF apoE levels were lower in AD patients than in controls. We conclude that the inheritance of the epsilon 4 allele of apoE is a risk factor for AD in the Finnish population.