We report a detailed study on oral lesions and their association with the WHO revised provisional case definition of AIDS as well as serologic signs of HIV infection among 186 patients in Dar Es Salaam, Tanzania. The patient material consisted of 39 hospitalized suspected AIDS patients, 44 medical nonsuspected patients, 53 dental outpatients, and 50 patients with sexually transmitted diseases. The male:female ratio was 2.1:1 on average. Oral examination was done without knowledge of the HIV status of the patients. Among 39 suspected AIDS patients 97% had WHO AIDS criteria and 90% were seropositive for HIV. Among the 147 patients not suspected of having AIDS 18 (12%) had AIDS criteria and 15% had serologic evidence of HIV infection. The presence of WHO AIDS criteria correlated significantly with the presence of HIV antibodies, but not with HIV antigen. Oral lesions were found in 54% of those with AIDS criteria and 52% of HIV-infected patients, as compared to 3% and 6% of the patients without AIDS criteria and HIV infection, respectively (p less than 0.01). Among patients with AIDS atrophic candidiasis occurred in 21%, pseudomembranous candidiasis in 23%, hairy leukoplakia in 36%, herpetic stomatitis in 2%, Kaposi's sarcoma in 4%, and nonspecific ulcer in 4%. The presence of oral lesions had a high predictive value for presence of AIDS criteria as well as for presence of HIV infection in this hospital setting. All patients should have a thorough oral examination and the presence of the aforementioned oral lesions should lead to testing for HIV infection.