A patient with known repaired complex congenital heart disease was referred as an emergency with increasing breathlessness on exertion. He was not short of breath at rest and had a saturation of 85% in air. A CT pulmonary angiography demonstrated decreased flow from his right ventricle to pulmonary artery, which was thought to be due to pulmonary embolism. We reviewed the CT with a knowledge and understanding of his anatomy and found that he had developed a false aneurysm of his right ventricular outflow tract, which required surgical treatment.