Diagnosis of oral hairy leukoplakia by ultrastructural examination of exfoliative cytologic specimens. Lingual exfoliative cytologic specimens (scrapings) were obtained from 18 patients positive for human immunodeficiency virus with clinical oral hairy leukoplakia. Buccal mucosal scrapings were obtained from 12 of these patients. The specimens were processed for examination by transmission electron microscopy (TEM). Sixteen (89%) of the lingual specimens revealed infection of keratinocytes by herpes-type virus. There was no evidence of virus infection in the 12 buccal mucosal scrapings. Fungal hyphae were seen by TEM in 14 (78%) of the lingual scrapings and two (17%) of the buccal scrapings. One exfoliative specimen and two biopsy specimens were stained for Epstein-Barr virus DNA with a DNA probe. The demonstration of herpes-type virions by TEM in keratinocytes from a lesion clinically suspected to be hairy leukoplakia provides direct, objective diagnosis. Furthermore, use of exfoliative cytologic specimens provides a clinically simple, noninvasive technique. 