It is now known that possession of one of the three common forms of the apolipoprotein E gene (allele 4) confers an increased risk for Alzheimer's disease (AD), both familial and sporadic, and that this risk is dose-dependent. Other genes that may play a role in AD, either through independent association with the disease or through modification of, or interaction with, the existing apolipoprotein E (APOE) risk, are now under investigation including the alpha-1-antichymotrypsin (ACT) gene, the very low density lipoprotein receptor (VLDL-R) gene, and the presenilin-1 (PS-1) gene. Kamboh et al. [1995] reported that a polymorphism in the โฃ-1-antichymotrypsin gene could modify the risk for AD conferred by the APOE locus, specifically by increasing the risk for AD among 4 homozygotes. The ACT gene, which is found on chromosome 14, has previously been proposed as a candidate for AD due to the presence of the ACT protein in senile plaques and the reported elevation of the protein in the cerebro-spinal fluid (CSF) and serum of AD cases. We have investigated this reported association within our familial and sporadic AD dataset, where we find no independent association between ACT and the occurrence of AD. Logistic regression analysis excludes ACT or the interaction between ACT and APOE as significant contributors in the prediction of disease status. By this analysis, ACT genotyping does not provide additional information about an individual's risk of Alzheimer's disease beyond the risk information conferred by APOE genotype alone. Am.