Pain or tenderness of an abdominal aortic aneurysm is widely believed to signify acute expansion and imminent rupture. To assess the potential benefit of emergency operation for the group of patients with an acutely expanding aneurysm, the clinical course of 19 patients with a symptomatic but unruptured expanding abdominal aortic aneurysm was compared with 117 patients undergoing elective abdominal aortic aneurysm resection, and 69 patients having operation for a ruptured abdominal aortic aneurysm. Postoperative morbidity was high in the patients with an expanding abdominal aortic aneurysm, and included a 21% incidence of myocardial infarction, a 10% incidence of stroke, a 37% risk of ventilatory failure, and a 31% incidence of acute renal failure, which was not statistically different from the results in patients having ruptured abdominal aortic aneurysm resection. Patients undergoing elective abdominal aortic aneurysm resection had only an 8% risk of myocardial infarction, and only a 2% risk of stroke, ventilatory failure, or renal failure. The mortality rate for expanding abdominal aortic aneurysm resection was 26% compared to 35% for ruptured abdominal aortic aneurysm (p = 0.31). Both emergency operations had a mortality rate more than five times greater than the 5.1% after elective procedures (p = 0.008). Our findings emphasize the need for early and aggressive treatment of abdominal aortic aneurysm in the elective setting, even in the patient at high risk, and suggest that the preoperative assessment and modification of risk factors is important to prevent the cardiac, cerebrovascular, pulmonary, and renal complications seen accompanying an emergency operation of this magnitude.