In a group of 61 patients admitted to New England Deaconess Hospital, Boston, with a diagnosis of acquired immune deficiency syndrome (AIDS), 25 were found to have Kaposi's sarcoma involving the skin and mucous membranes. Of these 25 patients, eight had lesions involving the respiratory system. Radiographically, patients with Kaposi's sarcoma had hilar and mediastinal adenopathy with perihilar parenchymal infiltration which progressed to diffuse bilateral infiltrates over a period of months. This pattern and the tempo of its evolution were distinctly different from the diffuse infiltrates seen in patients with Pneumocystis carinii pneumonia. Bronchoscopy was performed in seven of the eight patients, revealing characteristic lesions of Kaposi's sarcoma in the airways. We propose that parenchymal pulmonary Kaposi's sarcoma can be strongly suspected in a patient with AIDS who has the following features: a characteristic radiologic pattern; endobronchial Kaposi's sarcoma at bronchoscopy; and no evidence of opportunistic infection. In this subset of patients, further diagnostic intervention such as open lung biopsy, a procedure with potential morbidity in these ill individuals, may be unnecessary.