We previously reported a genetic association between the 3.5 kb (F) Taq I restriction fragment length polymorphism allele of the apolipoprotein CII gene on chromosome 19 and familial Alzheimer's disease. Here, we report an additional analysis of this association performed on an expanded and better defined data set of 23 families with familial Alzheimer's disease. The F allele frequency in affected family members in the expanded set was 0.62 +/- 0.06 (mean +/- standard error, n = 51 subjects), which differed significantly from a frequency of 0.39 +/- 0.02 (n = 226) for unrelated control subjects (Z = 3.75, p less than 0.0002). These results are consistent with our previous findings and suggest an association between the F allele of apolipoprotein CII and familial Alzheimer's disease. When the apolipoprotein CII locus was tested for linkage to familial Alzheimer's disease, LOD scores summed for the complete group of families were negative and close linkage was excluded. Close linkage was also excluded for early-onset families (mean onset age less than or equal to 60 years), but small positive LOD scores were obtained for late-onset kindreds.