The authors present their study of 10 patients affected by Alzheimer's disease and 10 patients with multinfarctual dementia, who are already part of a CNR study, senile dementia project, longitudinal study. The method adopted was that of DNA crossbreeding and amplification, so as to have a precise individualisation of alleles APo-E, Apo E-3, Apo E-4 and of their various phenotype combinations. These isoforms are evaluated and compared with the disease studied, in order to help to identify a timely-marker for senile dementia. On the results obtained we hypothesised an association between Apo E-4 and the disease of about 40% for Alzheimer's disease, which leads us to increase our number of cases, regarding associations between demential diseases either Alzheimer or vascular and the presence of Apo E-4.