"Mycotic" aneurysm was originally described by Osler in 1885. It occurs in a normal or atherosclerotic artery from septic emboli in patients with infective endocarditis. However, now the term "mycotic" aneurysm is applied to all cases of aneurysms caused by any organisms. From September 1988 to November 1990, four cases of ruptured mycotic aneurysm were diagnosed at our institute. Three were males and one was a female; they were elderly with atherosclerosis of the aorta. The diagnosis was established by computed tomography (CT) scan, bacteriology or operative findings. Two of the patients underwent emergency operation; only one survived. In general, the diagnosis of mycotic aneurysm is based on the classical features of fever, abdominal or chest pain, positive blood culture and a pulsatile mass. Because the clinical manifestations are often variable, a patient may present with chronic sepsis (esp. Salmonella sp) of unknown origin with deterioration to a fatal outcome from the aneurysmal rupture, which is a rare cause of retroperitoneal abscess or pericardial effusion. The principles of management, including high clinical suspicion, an accurate diagnosis by imaging studies (arteriography or CT scan), prolonged effective antibiotic therapy, arterial ligation or wide excision of the infected lesion, intraoperative Gram's stain and culture, extra-anatomic bypass grafting through clean tissue planes, and prolonged postoperative follow-up, are indispensable to reduce morbidity and mortality.