Apolipoprotein E (apoE) is associated with Alzheimer's neurofibrillary tangles and beta-amyloid protein in senile plaques. It also appears to play an important part in the redistribution of lipids that follows deafferentation and neurodegeneration in the brain. The gene for apoE is on chromosome 19, within the genomic region previously associated with late-onset familial Alzheimer's disease (AD). We have studied apoE phenotype expression and the corresponding allele frequencies (epsilon 2, epsilon 3, epsilon 4) in 91 patients with sporadic AD and 74 controls. There was a significant association between epsilon 4 and sporadic AD (epsilon 4 frequency 0.380 in AD and 0.122 in controls, p < 0.01). Analysis of epsilon 4 in whom AD develops this tended to happen earlier in life than in those with epsilon 3 or epsilon 2. The epsilon 4/AD association was more pronounced in women. Octogenarians with AD had an epsilon 4 allele frequency that was 3 times higher than one reported, in a different study, in healthy octogenarians. ApoE may be an important susceptibility factor in the aetiopathology of sporadic AD.