An isolated iliac aneurysm is an uncommon entity with a relative incidence to abdominal aortic aneurysm of 0.9 to 1.9 percent. Analysis of 16 cases with 20 isolated iliac aneurysms experienced at our vascular service makes the basis of this report. Seventeen aneurysms involved the common iliac artery, two the internal iliac artery and one the external iliac artery. As we experienced 341 abdominal aortic aneurysms during the same period, relative incidence of isolated iliac aneurysm to abdominal aortic aneurysm was 4.7 percent. Fifteen of the 16 cases were male; isolated iliac aneurysms had significantly higher preponderance for the male. Six of the cases were presented with ruptured aneurysms, giving a rupture rate of 37.5 percent. This rupture rate was significantly higher than that of abdominal aortic aneurysms of 11.1 percent. Operation was performed on 15 patients; the remaining one died before surgery. Although there was no operative death among the elective cases, three patients in the ruptured group died postoperatively giving a mortality rate of 60 percent. The least diameter of the ruptured aneurysms was 3.5 centimeter. Among the 16 patients 5 presented with a ruptured aneurysm and 3 had no symptoms related to the aneurysm. Five patients complained of a pulsatile abdominal mass. Two of the ruptured aneurysms were associated with an iliac arteriovenous fistula. Both of them involved the common iliac artery. Although emergency operation was performed on one of them, he died of acute renal failure due to preoperative dehydration and hypovolemia. The another one was operated on electively after intensive medical treatment for heart failure and the patient survived.(ABSTRACT TRUNCATED AT 250 WORDS)