Studies regarding different viruses of the herpes family, such as cytomegalovirus (CMV), Epstein-Barr virus (EBV), or human herpes virus 6 (HHV-6) in Alzheimer's disease (AD) are scarce. DNA from peripheral blood leukocytes (PBL) and brain samples were analyzed for the presence of CMV, EBV, or HHV-6. All samples were negative for CMV. EBV positivity was 6% in AD brains, whereas 45% of PBL samples from AD patients and 31% from controls were positive for EBV (p = 0.05). HHV-6 showed a 23% positivity in PBL samples from AD and 4% from controls (p = 0.002). 17% of AD brains were HHV-6 positive. Within a group of elderly individuals, followed up for 5 years, EBV-positive or HHV-6-positive PBL increased in those who developed clinical AD. Virus serological positivity was also investigated, and IgG levels for CMV and EBV antigens were also increased in those subjects who developed AD during the follow-up. Our findings suggest that EBV and HHV-6 may be environmental risk factors for cognitive deterioration and progression to AD in elderly persons.