Approximately 30% of patients with fatal head injuries have deposits of amyloid beta-protein (A beta); these are predominantly individuals carrying the epsilon 4 allele of apolipoprotein E (apoE). A beta deposition occurs in Alzheimer's disease (AD), for which approximately 50% of the genetic risk is attributed to apoE epsilon 4. The 1,1 genotype of a presenilin-1 (PS-1) polymorphism has been suggested to account for about half of the remaining genetic risk for AD. We related the PS-1 genotypes of 90 head-injured individuals to A beta deposition and apoE genotype. There was no difference in PS-1 genotype or allele frequencies between individuals with and without A beta deposits. Eighteen of 23 individuals with A beta deposits had apoE epsilon 4 as a risk factor. Three of five individuals without apoE epsilon 4 had the PS-1 1,1 genotype. If PS-1 genotype influences A beta deposition the effect is small and is overwhelmed by that of apoE genotype.