Most aneurysms of the isthmus aortae, particularly those following thoracic trauma, are most likely to be diagnosed as acute dissecting or post-traumatic pseudoaneurysms. Furthermore, penetrating atherosclerotic ulcers in patients with atherosclerosis, congenital aneurysms such as ductus diverticulum and Kommerell's diverticulum, luetic aneurysms, and saccular aneurysms associated with Marfan's syndrome have to be included in the list of differential diagnoses. In view of the severe effect of any open thoracic surgical intervention, exact preoperative diagnosis is crucial. We report the case of a 73-year-old male patient who was accidentally diagnosed with an aneurysm of the atherosclerotic isthmus aortae. The aneurysm extended from the aorta to the dorsal site. The sacciform aneurysm (4.5 cm) was resected electively. Based upon localisation and pathomorphological findings, a penetrating ulcer was diagnosed. We also present a review of the current literature and give a survey of the differential diagnoses of aneurysms of the aortic isthmus: penetrating atherosclerotic ulcus, acute (traumatic) dissecting aneurysm, post-traumatic pseudoaneurysm, ductus diverticulum, Kommerell's diverticulum, syphilitic aneurysm, and sacciform aneurysm due to Marfan's syndrome.