Histopathologic changes in the lung in a patient with Behcet's disease are described and the literature dealing with pulmonary pathologic changes in this disease is summarized. The basic lesion is a lymphocytic and necrotizing vasculitis involving all sized pulmonary arteries, veins and septal capillaries. Complications include aneurysms of elastic pulmonary arteries, arterial and venous thromboses, pulmonary infarcts, and bronchial erosion by pulmonary artery aneurysms. Striking periadventitial fibrosis develops and is believed to be related to repetitive vascular inflammatory insults. Peculiar newly formed collateral vessels, lacking elastic lamellae and derived from smooth muscle metaplasia around arterioles, are found in the periadventitial fibrous tissues around thrombosed arteries and aneurysms. Cases of pulmonary artery aneurysms of obscure origin, including those found in the Hughes-Stoven syndrome, are reviewed and both clinical and pathologic findings in these reports are found to be remarkably similar to those observed in Behcet's disease. It is suggested that such cases represent unrecognized or incomplete expressions of Behcet's disease.