Mycotic aneurysms as defined in this study include only those naturally occurring aortic aneurysms that result from or are secondarily infected by bacteria arising in a distant site of infection. Of the 2,585 patients treated for aortic aneurysm during the past 8 1/2 years, 22 patients had disease conforming to this definition. The aneurysms were located in the ascending aorta in 2 patients, ascending aorta and arch in 5, arch and descending aorta in 1, descending thoracic aorta in 1, separate descending and abdominal aorta in 1, thoracoabdominal aorta in 5, upper abdominal aorta in 6, and infrarenal abdominal aorta in 1. The primary source of infection was the urinary tract in 2 patients, salmonellosis in 4, pneumonia in 3, sub-acute bacterial endocarditis in 2, ear, nose, and throat in 2, cellulitis of the hand in 1, chronic wounds in 2, dental extraction in 1, lumbar disc space infection in 1, septic thrombophlebitis in 1, and generalized febrile illness in 3. The duration of febrile illness ranged from 2 weeks to 1 year. All patients were treated with antibiotics and operation was performed within 24 hours after admission in 11 patients and within one to eight days after admission in 11. Treatment consisted of in situ graft replacement. Appropriate antibiotics were given intravenously for 4 to 6 weeks in patients with positive cultures and continued orally for the rest of the patients' lives. Of the 22 patients, 19 (86%) were early survivors, and all are still alive 3 months to 8 years postoperatively. Only 1 had a recurrent infection, which involved the intervertebral disc space.