We reviewed the clinical, radiographic, and histologic features of nine patients with AIDS and pulmonary disease due to Mycobacterium avium complex (MAC). Pulmonary MAC disease was defined by (1) the isolation of MAC from two or more lower respiratory tract specimens or from a single lung biopsy sample, (2) an infiltrate revealed by chest radiography, and (3) the absence of other identified pulmonary pathogens or malignancies. Pulmonary MAC disease was present in five (2.5%) of 200 patients with disseminated MAC infection and in four additional patients without evidence of dissemination, as assessed by blood culture. The median CD4 cell count at the time of presentation was 90/microL. Pulmonary MAC disease was the initial AIDS-defining infection in five patients and presented within a median of 5 months after the initial infection in four patients. Radiographic patterns for these nine patients included consolidating or nodular infiltrates and cavitation. The histopathology of pulmonary MAC disease was characterized by granulomatous inflammation, often associated with necrosis and few evident organisms. The conditions of all patients treated with multidrug regimens clinically improved.