Among 72 patients with Behçet's syndrome, seven had pulmonary vascular involvement. Additional data from 42 cases in the literature are discussed. Recurrent episodes of dyspnea, cough, chest pain, and hemoptysis were the primary clinical signs, mainly in young men, appearing 3.6 years after the first manifestation of Behçet's syndrome. Fever, elevated ESR, and anemia were common, and chest x-ray films showed pulmonary infiltrates, pleural effusions, and prominent pulmonary arteries. Ventilation-perfusion scans showed perfusion defects even when chest x-ray films were normal. Pulmonary artery aneurysms were seen in 7/13 in whom angiography was done. Of 42 patients, 16 died, 15 from fatal pulmonary hemorrhage, 80 percent within two years from the development of pulmonary disease. Histopathologic study results showed vasculitis of pulmonary vessels of various sizes, leading to thrombosis, destruction of the elastic laminae, aneurysms, and arteriobronchial fistula. In addition, pulmonary emboli and the aphthous lesion of the tracheobronchial tree may aid the clinical picture. Anticoagulant therapy may be hazardous in patients with aneurysmal dilatation of the pulmonary vascular tree, and the beneficial effect of corticosteroid therapy is discussed. Pulmonary vasculitis in Behçet's syndrome is a unique clinical and pathologic picture, differing from other vasculitides affecting the lung, presents a major threat to the patient's life.