In the course of the infection with the human immunodeficiency virus (HIV), we frequently observe disorders of the mucous membranes and, occasionally, they present the first manifestation of HIV-induced immunodeficiency. Like in other organs, opportunistic infections and malignant tumors prevail as a result of the impaired immune system. Opportunistic infections are characterized by frequency (candidiasis), aggressive expansion, persistence, frequent recurrences, and resistance to therapy (gingivitis, parodontitis, herpes simplex, warts). Oral hairy leucoplakia is considered a specific lesion of HIV infection. Malignant tumors, such as Kaposi's sarcoma, non-Hodgkin's lymphoma, and squamous cell carcinoma, may cause marked morbidity in AIDS patients; occasionally, the clinical picture of Kaposi's sarcoma and non-Hodgkin's lymphoma is rather uncharacteristic. Other manifestations on the mucous membranes may arise in association with systemic reactions, such as drug eruptions, thrombocytopenic purpura, or acute HIV infection. The etiology of still other lesions of the mucous membranes (e.g. chronic recurrent ulcers, xerostomia, disorders of pigmentation) is incompletely understood. The awareness of these disorders of the mucous membranes in HIV infection is of diagnostic, therapeutic and epidemiological importance.