People with deficient cell-mediated immunity have an increased susceptibility to viral infections and certain cancers, particularly non-Hodgkin's lymphomas and cancers of the skin and anogenital region. These are linked to viral origins. Anogenital neoplasms in immunodeficient patients show a strong association with HPV infection; often occur at relatively young ages; involve multifocal locations; and tend to persist, recur, and progress rapidly, despite standard therapy. Because standard therapy of anogenital HPV lesions and neoplasia is often not effective in immunodeficient patients (and others with an anogenital neoplastic syndrome), special treatment is required. 5-Fluorouracil chemosurgery, followed by maintenance 5-fluorouracil therapy, is often effective and provides field suppression against recurrent HPV infection and neoplasia, with minimal damage to affected organs. After removal of all detectable HPV and neoplastic lesions, immunodeficient patients require close surveillance of the entire anogenital tract. Immunodeficient patients are an in vivo human laboratory in which to study the natural history of HPV and its oncogenic effects on the anogenital tract. The theory of HPV oncogenesis is supported by the evidence gathered from these patients.