Since the original description of oral hairy leukoplakia among homosexual men in San Francisco in 1984, this white lesion of the tongue has been seen in the mouths of persons infected with the human immunodeficiency virus (HIV) worldwide. Its presence in HIV-positive persons usually but not always indicates fairly rapid progression to acquired immunodeficiency syndrome in the absence of antiretroviral therapy. Although the lesion appears to be common in HIV-positive persons, it is also, albeit rarely, seen in other conditions associated with immunosuppression. Epstein-Barr virus is associated with and presumably causes hairy leukoplakia, and the lesion offers insights into the biology of this ubiquitous DNA-oncogenic virus.