The radiographic and pathologic features of 13 cases of mycotic aneurysm of the aorta were reviewed retrospectively. In four cases the mycotic aneurysm was associated with bacterial endocarditis following aortic valve replacement, and in nine cases it was associated with spontaneous bacterial endocarditis. Postmortem examination revealed that the mycotic aneurysm was most frequently found in cases with bicuspid aortic valves. In all cases the chest radiograph revealed cardiomegally, usually with pulmonary vascular congestion. In the six patients in whom the diagnosis was established angiographically, the aneurysm was manifested by an irregular saccular collection of contrast medium under one of the coronary arteries. Mycotic aneurysms must be differentiated from other lesions with similar angiographic findings, such as a congenital aneurysm of the sinus of Valsalva, prolapse of an aortic cusp through a membranous ventricular septal defect, and a congenital aortic-left ventricular tunnel.