Clinical and morphologic observations are described in two patients with severe pulmonary arterial hypertension without pulmonary venous hypertension from fibrosing mediastinitis. In one patient, both main pulmonary arteries and one major pulmonary vein were severely narrowed by dense fibrous tissue; in the second patient, only the right main pulmonary artery was severely narrowed. Both patients had normal intrapulmonary arteries and normal pulmonary parenchyma. Of nine previously described necropsy patients with pulmonary hypertension due to fibrosing mediastinitis, seven had severe narrowing of multiple large pulmonary veins and in six of them the pulmonary hypertension was entirely due to pulmonary venous obstruction. In one other patient, the pulmonary hypertension was due to obstruction of one main pulmonary artery and several large pulmonary veins. Each of these seven previously described patients had severe changes in the small intrapulmonary arteries. Of the other two previously described patients with pulmonary hypertension from fibrosing mediastinitis, one had severe narrowing of only the main right pulmonary artery, and the other, of both main pulmonary arteries. Thus, although pulmonary arterial hypertension in patients with fibrosing mediastinitis is usually due to obstruction of multiple large pulmonary veins and to severe secondary changes in small intrapulmonary arteries, fibrosing mediastinitis can cause severe pulmonary hypertension by obstructing the right or both main pulmonary arteries.