Pulmonary Kaposi's sarcoma is an unusual pre-mortem diagnosis in AIDS patients. During a four-year period, 318 patients with AIDS-associated Kaposi's sarcoma presented to New York University Medical Center; 19 patients were found to have pulmonary involvement. All 19 patients were homosexual men; 18 of these patients presented with cutaneous and/or visceral Kaposi's sarcoma. Seventy-nine percent of patients had previous or concurrent pulmonary opportunistic infections. Clinical presentation of pulmonary Kaposi's sarcoma was indistinguishable from opportunistic pneumonia with respect to symptoms, physical examination results, and laboratory findings. Chest roentgenograms revealed a typical nodular pattern in only five patients. In contrast to AIDS-associated opportunistic pneumonia, the yield from fiberoptic bronchoscopy (24 percent) and open lung biopsy (56 percent) was low. Pathologically, Kaposi's sarcoma of the lung is less cellular than the cutaneous lesion. It is focal and scattered throughout the pulmonary interstitium, which may explain the difficulty in pre-mortem diagnosis.