Three patients who were seropositive for human immunodeficiency virus underwent surgery for infected aneurysm of the abdominal aorta. Fever and abdominal pain were the principal presenting clinical features. None of the patients had any opportunistic infections or endocarditis. In two cases, a ruptured aneurysm was demonstrated radiographically. In the remaining case, sonograms were diagnostic. The organisms responsible were salmonella, Hemophilus influenzae, and Mycobacterium tuberculosis. In two cases, the infectious origin was evidenced by bacteriologic examination of the aortic wall, which revealed the presence of Salmonella enteritidis and Koch's bacillus. Although Hemophilus influenzae was not found in the aortic wall of the remaining case, the infectious origin of the aneurysm was established because preoperative blood cultures were positive for this pathogen, and pathohistologic examination of the specimen showed destruction associated with leukocyte infiltration of the aneurysmal wall. An in situ prosthetic graft replacement protected by omentum was performed in all three cases. Antibiotic therapy was continued for several weeks. All patients are well with follow-up ranging from 10 to 21 months. Infectious aneurysm associated with human immunodeficiency virus seropositivity results in bacterial infestation of an atheromatous aorta. Infected phenomena are promoted by cellular immunodeficiency. Surgery was justified in these cases because of the immediate threat of rupture.