The role of Aspergillus species as a pathogen in acquired immunodeficiency syndrome (AIDS) has not been clearly defined. From 1984 to 1989, more than 2,000 AIDS patients were seen at Beth Israel Medical Center, New York. Aspergillus was isolated in ten patients; seven had invasive disease and three had noninvasive disease. Invasive pulmonary aspergillosis (IPA) was diagnosed in six patients and invasive renal aspergillosis was found in one patient. Five were homosexual men and two were intravenous drug users. At presentation, all ten had fever, seven had cough, eight had dyspnea, and five had pleuritic chest pain. Chest roentgenograms revealed focal infiltrates in six patients, bilateral interstitial infiltrates in two patients, and bilateral pneumothoraces in one patient. Predisposing conditions included corticosteroid therapy in four, granulocytopenia (less than 1,000/cu m) in two, and broad-spectrum antibiotic therapy in five. Three of the four patients receiving corticosteroids received them as adjuvant therapy for Pneumocystis carinii pneumonia (PCP). Aspergillus was identified antemortem in eight patients, in bronchoalveolar lavage (BAL) fluid in six, in transbronchial biopsy specimen in three, in open lung biopsy specimen in one, and postmortem in one patient. Six of seven patients had at least one concomitant pulmonary process. Six underwent necropsy and findings showed IPA in three, disseminated aspergillosis in two, and PCP in one. Invasive aspergillosis, although significant, is uncommon in AIDS. When Aspergillus is isolated in the setting of corticosteroid therapy, antibiotics, or granulocytopenia, one must suspect invasive disease.