This report describes a case of right pulmonary artery obstruction caused by a dissecting aneurysm of the thoracic aorta. Initial ventilation-perfusion (V/Q) scan revealed no perfusion to the right lung. Lack of right lung perfusion plus other clinical abnormalities led to an initial diagnosis of massive embolism in the right lung. Bilateral pulmonary arteriography of the thoracic and abdominal aorta revealed extrinsic compression of the right pulmonary artery near its origin and the suggestion of an ascending aortic dissection. Subsequent ascending aortography and computed tomography (CT) confirmed a dissection of the ascending aorta. Aortography in the true lumen of the aorta revealed trivial aortic insufficiency. These diagnostic procedures confirmed the presence of an acute dissecting aneurysm of the ascending aorta that caused compression of the right pulmonary artery. The finding of a massive unilateral segmental defect with normal ventilation upon lung scanning does not always ensure a diagnosis of acute pulmonary embolus. Patient presentation with massive unilateral perfusion defect merits further evaluation with pulmonary arteriography.