Twelve patients with rupture of the perivisceral abdominal aorta were admitted to the UCLA Medical Center between 1984 and 1996. Six patients had atherosclerotic thoracoabdominal aneurysms (TAA) which ruptured in the visceral segment of the aorta. The remaining 6 patients proved to have ruptured mycotic aneurysm (MA). Clinical presentation was different in the two groups. Whereas all 6 patients with TAA and < 24 hr history of abdominal, chest, or back pain, patients with MA had these symptoms for 2-5 weeks (mean 3.4 weeks). History of sepsis was present in 4/6 MA and in 0/6 TAA patients. No difference in risk factors for atherosclerosis were seen between these two groups. Clinical outcomes were also different. Operation consisted of in situ vascular grafting in all patients. Operative mortality for TAA was 33% (2/6), whereas all patients with MA survived repair with no operative mortality. Two patients had cardiac arrest prior to surgery. One of these had a TAA and died 5 days after surgery, whereas the other survived repair of an MA. Follow-up ranges from 1-84 months (mean 48 months). Four survivors in the TAA group are alive at 6, 8, 14, and 84 months, with the latter having a pseudoaneurysm of the visceral patch-graft anastomosis. All 6 patients with MA are alive at 1-73 months (mean 39 months) without evidence of graft sepsis or recurrent aneurysm. We conclude that rupture of the visceral portion of the aorta is often associated with a mycotic process, with important differences noted in clinical presentation when compared to atherosclerotic TAA. Surgical intervention is effective in both MA and TAA. Operative mortality, however, is significantly higher in patients with ruptured TAA. In situ prosthetic replacement for ruptured MA is associated with low mortality and excellent long-term results.