In order to identify the genetic locus responsible for familial dementia of the Alzheimer type (DAT), we are studying 10 families in which DAT appears to be inherited as an autosomal dominant trait. Genotypes for a TaqI restriction fragment length polymorphism (RFLP) at the apolipoprotein CII locus were determined for the following groups: affected and unaffected DAT family members, DAT subjects with no family history of the disease, and normal control subjects. The control group included 103 individuals from our study and 123 from the study of Wallis et al.(Hum. Genet., 68 (1984) 286). The frequency of the TaqI fast (F) allele in the affected familial DAT subjects (0.64±0.08) differed significantly from that for the control group (0.39±0.02) (Z = 2.87,P0.40). Subsequently, genotypes were determined for two other polymorphisms at the Apo CII locus: a BanI RFLP and a Bgll RFLP. For these two polymorphisms, the allele frequencies for the familial DAT subjects differed from the unaffected control groups but the differences were smaller and not statistically significant. These data suggest a previously unrecognized association between the Apo CII TaqI F-allele and familial DAT.