Alpha-2-macroglobulin (A2M) is a proteinase inhibitor that is present in senile plaques and may play a role in metabolism of amyloid beta (A beta) peptide. Recently it was reported that inheritance of the deletion allele (A2M-2) confers increased risk for late-onset Alzheimer disease (AD) with significance of this effect similar to the epsilon4 allele of apolipoprotein E (APOE). We examined the distribution of A2M genotypes and alleles in a cohort of 146 AD patients and 160 age-matched non-demented individuals. There was no evidence for association in the total sample or in subsets stratified by age or APOE epsilon4 status. These results suggest that this polymorphism is not a strong genetic risk factor for either early- or late-onset forms of the disorder. However, they do not exclude the possibility that an AD susceptibility allele is located elsewhere in A2M or a nearby gene.