Previous work suggests an association between allele 1 and the 1-1 genotype of an intronic polymorphism in the presenilin-1 (PS-1) gene and late onset Alzheimer's disease. We found an excess of the 1-1 genotype in our late onset clinical sample (p = 0.006, one-tailed) but not in our postmortem confirmed sample, which instead exhibited an excess of allele 1 (p = 0.02, one-tailed). No interaction between PS-1 and ApoE genotype was detected and the findings remained significant when the effects of ApoE were taken into account (p = 0.03, one-tailed). These results suggest that the PS-1 polymorphism, or a locus in linkage disequilibrium with it, acts as a risk factor for late onset AD.