Syphilis can lead to saccular aneurysms of the thoracic aorta. Today syphilitic aortic aneurysms are rare. The average time from primary infections to the development of aortic aneurysms is 10 to 15 years. An 83-year-old man was admitted with a giant aneurysm of the descending thoracic aorta. The patient had first experienced subacute pain in the left hemithorax some weeks previously. Computer tomography scan detected an 11 × 11 cm aneurysm of the descending aorta. Serodiagnostic tests for syphilis were highly positive. Femoro-femoral bypass was initiated and a tube graft was interposed. The postoperative course was uneventful, the patient was discharged at the twentieth postoperative day. Histological examination of the aneurysmal wall showed typical syphilitic changes. Postoperatively, Penicillin G was given for 6 months. Three years later the patient remains asymptomatic. Although extremely rare today, tertiary syphilis should be considered in the differential diagnosis of thoracic aneurysms. In selected octogenarians replacement of the descending aorta is possible.