Aneurysm of the ductus arteriosus is a very rare congenital lesion in adults that can be associated with thromboembolism, rupture, and death. Its detection in a silent clinical phase is very important for planning appropriate treatment and avoiding potentially fatal complications. We report a case of a patent ductus arteriosus aneurysm of very large size (65.5 mm) that was incidentally discovered with low-dose (3.2 mSv) multidetector computed tomography in an asymptomatic 67-year-old man. The presence of coronary disease was also ruled out with this non-invasive imaging modality. Further evaluation with echocardiography and selective angiography confirmed the diagnosis. At surgery, the aneurysm was successfully resected via a left posterolateral thoracotomy.