The finding of a massive unilateral segmental defect with normal ventilation upon lung scanning does not always secure the diagnosis of acute pulmonary embolus. We present a patient whose ventilation-perfusion lung scan suggested a significant embolic phenomenon, but who was subsequently found to have compression of her right pulmonary artery by a dissecting aortic aneurysm. Conditions that present with a unilateral perfusion defect merit further evaluation with pulmonary arteriography.