Analytic epidemiology has contributed significantly to the generation and testing of hypotheses of the causes of AD. Several case-control studies have indicated risk factors related to the genetic hypothesis, such as: the presence of cases of either AD or Down's syndrome in other family members and the advanced age of the mother at subject's birth. In this respect recent molecular biology studies on DNA from patients affected by the familial form of AD, have demonstrated a genetic polymorphism localized on chromosome 21. On the same chromosome, the gene coding for beta-amyloid has been also recently localized. Immune, viral and toxic factors, thought to cause AD, have been also investigated in case-control studies, none of them has been found consistently associated with the disease, with the only exception of the head trauma. On the other hand most case-control studies have been carried out in younger cases and no large studies are yet available for late onset patients.