Several organs are severely affected in human immunodeficiency virus type 1 (HIV-1) infection. One of these is the oral cavity, in which a variety of lesions of known and unknown causes are observed. The possible causes of some oral lesions and the presence of bacteria, virus and yeasts in saliva and gingival crevices have been investigated in patients infected with HIV-1. Common oral symptoms, which are strongly over-represented in adult patients infected with HIV-1, are Candida infection, necrotic periodontal disease, hairy leukoplakia, dry mouth problems, oral Kaposi's sarcoma and oral ulcers. These symptoms are seen mostly in immunocompromised HIV-1-infected patients with other HIV-1-related symptoms. Other less common oral lesions in adult patients infected with HIV-1 are infection with herpes simplex virus, varicella-zoster virus and papilloma virus, malignant lymphoma, salivary gland disease, drug-related ulcers or ulcers infected with microorganisms such as mycobacteria and cytomegalovirus. Changes in the bacterial flora and HIV-1 itself in the oral cavity do not explain the observed lesions. Fungal and viral infections are common in HIV-1-infected patients. Epstein-Barr virus is associated with oral hairy leukoplakia, and cytomegalovirus (CMV) may be correlated to some oral symptoms such as necrotizing gingivitis and dry mouth problems. The shedding of Epstein-Barr virus may indicate early deterioration of the cellular immune response, while CMV appears to be related to late HIV-1-induced symptoms.