Hairy leukoplakia is a specific oral lesion associated with the opportunistic development of Epstein-Barr virus in the oral epithelium. It is now considered to be an early sign of HIV-induced immunosuppression. Four cases of oral hairy leukoplakia (OHL) from the lateral borders of the tongue of male AIDS patients were investigated by transmission electron microscopy. At the ultrastructural level, herpes-like viral particles were detected in the oral lesions of all cases. Indirect immunofluorescence performed on two cases showed the presence of EBV antigens in the nuclei and the cytoplasm of the infected epithelial cells. None of the specimens contained ultrastructural evidence of human papillomaviruses.