A previous study reported an unexpected increased frequency of the uncommon C4B2 allele of complement in a group of patients with senile dementia of the Alzheimer type. We compared the frequency of various C4B types in 25 patients with familial Alzheimer dementia (AD), 22 patients with sporadic AD, and 360 control individuals. Neither Alzheimer group showed any significant increase or decrease in the frequency of C4B alleles compared with controls. We were unable to confirm any association between AD and the C4B2 allele.