A 54-year-old man was hospitalized because of progressive shortness of breath and a diffuse pulmonary infiltrate in a radiograph of his chest. His pulmonary dysfunction was characterized by a selective loss of vital capacity in comparison with residual volume, a marked right-left shunt, and a rapidly deteriorating pulmonary diffusing capacity for carbon monoxide. the diagnosis of alveolar cell carcinoma was established before death by light and electron microscopic studies of material obtained by lobar lavage with a cuffed bronchographic catheter after sputum and bronchoscopic washings had failed to reveal malignant cells.