Sinusitis is commonly occurring in patients infected with Human Immunodeficiency Virus I (HIV), but the occurrence and etiology have not been established. The purpose of this study was prospectively to determine the occurrence, site and type of paranasal sinus abnormalities seen on MRI in HIV-infected patients with fever, to relate the abnormalities to clinical and immunological parameters, and to determine the microbiological agents found in the sinus aspirates. MRI was performed in 54 HIV-infected patients with 70 evaluable episodes of fever. Patients receiving antibiotics were excluded. Bactrim was permitted, when given as prophylaxis of Pneumocystis carinii pneumonia. If abnormalities were found on MRI, sinus aspiration was performed and the aspirate investigated. MRI abnormalities were found in 54.3% of the patients with a significantly higher occurrence of pathological changes in AIDS patients compared with HIV-infected without AIDS. In approximately 2/3 of the aspirates a probable, etiologic agent was found. However, 1/3 of these agents were atypical such as cytomegalovirus and mycobacteriae; in one patient Non-Hodgkin's lymphoma was found. The high occurrence of sinusitis in HIV-infected patients and the atypical findings in the sinus aspirates stress the importance of searching for sinusitis and the etiology to ensure the correct treatment.