To investigate whether brain tissue is infected latently by adenovirus via a monocyte/microglia-mediated entry mechanism, brain tissue resected at necropsy from seven senile subjects (five with senile dementia of Alzheimer type (SDAT) and two subjects without pathological changes) was examined for adenovirus early region 1A (E1A) gene and its expression using in situ hybridisation and immunohistochemical staining. HLA-DR positive, reactive microglial cells in both SDAT and normal brain tissue showed positive hybridisation and immunoreactive expression of adenovirus E1A. Thus there may be monocyte/microglia-mediated entry of adenovirus in the central nervous system which would be a novel and presumably common interaction between brain tissue and adenovirus.