A case of an ascending aortic dissection occurring in a patient 8 years after an aortic valve replacement is presented. The patient's initial clinical presentation was thought to be due to pulmonary embolism, and a ventilation- perfusion lung scan demonstrated mismatched absence of perfusion to the entire right lung. Aortography and findings at surgery demonstrated a type A dissection of the ascending aorta. The anatomic relationship of the aorta and pulmonary artery and their common tunica adventitia make the right pulmonary artery susceptible to extrinsic compression after aortic dissection. Although uncommon, dissection of the ascending aorta needs to be considered in patients who have undergone previous aortic valve surgery and demonstrate mismatched unilateral absence of perfusion to the right lung. Because anticoagulation is contraindicated in patients with acute aortic dissection, confirmation of the diagnosis with angiography usually is required.