Nine patients examined by arteriography were shown to have mycotic aneurysms involving the thoracic aorta, subclavian artery, renal artery, middle cerebral artery, hepatic artery, and splenic artery. Patients presented with sepsis, chest pain, mediastinal mass, headache, hypertension, and intraperitoneal bleeding. Etiologic factors included endocarditis, septicemia, drug abuse, and poorly controlled soft-tissue infection. Most mycotic aneurysms were virulent processes with rapid progression and only three of the nine patients (33%) survived. Since mycotic aneurysms may be associated with rapid progression and poor prognosis, early recognition is mandatory.