From August 1986 to October 1987, there were 5 cases of primary mycotic aneurysm of the lower abdominal aorta in Chang Gung Memorial Hospital at Kaoshiung. All patients were proved to have Salmonella cholerasuis (Sal. chol.) septicemia by blood culture. The ages ranged from 60 to 80 years old, the mean age was 71.6 years old. The male to female ratio was 4 to 1, 3 patients had diabetes mellitus (DM) and 3 had hypertension. The duration of symptoms lasted from 1 week to 2 months before diagnosis. Clinically, all patients had sepsis with fever, chills, leucocytosis, and complained of pain in the lower abdomen (80%), at flank (20%) or low back (20%). Abdominal tenderness was present in 3 (60%). Two patients underwent surgery, 1 expired during the operation, the other expired 1 month after operation because of retroperitoneal abscess and sepsis. Three were discharged in septic shock and expired within 1 day. The mortality rate was 100%. The diagnosis of complicated aneurysm of the lower abdominal aorta was established in all by computed tomography (CT). In conclusion, when there are clinical manifestations of sepsis, positive blood culture for Sal. chol., and pain or tenderness in the lower abdomen, flank area or back, one should consider the possibility of mycotic aneurysm of the lower abdominal aorta. Although the prognosis is poor, early surgical intervention may improve the outcome. And the diagnosis is best established by CT.