Skin and mucous membranes including the oral mucosa are among the preferential locations of opportunistic infections and secondary neoplasms in patients infected with the human immunodeficiency virus (HIV). Infections of the oral mucosa such as thrush occur in a high percentage of AIDS patients, patients with AIDS-related complex or HIV-seropositive individuals. The clinical appearance of the infections (herpes virus infection, periodontitis) is often marked by aggressive expansion, frequent recurrences or resistance to therapy. Oral "hairy" leukoplakia is considered to be a characteristic lesion in HIV-infected individuals. Tumors like Kaposi's sarcoma, squamous cell carcinoma and non-Hodgkin lymphoma of the oral mucosa may cause marked morbidity in AIDS patients. Such oral lesions are frequently the first indication of an HIV-infection. Dentists should be aware of the oral manifestations of HIV-infection and initiate diagnostic and therapeutic measures in the interest of the patients and for epidemiologic reasons.