The natural history of the ectatic infrarenal aorta remains undetermined. While surveillance protocols have been proposed for true aneurysms, no recommendations are currently available for ectatic aortas. The purpose of this study is to define the natural history and recommend surveillance protocols for the infrarenal aorta 2.5-2.9 cm in diameter. Screening of 12,500 yielded 223 patients with an infrarenal abdominal aorta 2.5-2.9 cm in diameter at a universityaffiliated VA medical center. All patients were prospectively followed by ultrasound from August 1993 through October 2000 and expansion rates were calculated by comparing the first and last study. Multivariate analysis of risk factors classically associated with abdominal aortic aneurysm was performed. Current data suggest that ectatic infrarenal aortas expand slowly, do not rupture, and rarely meet criteria for operative repair. No risk factors linked to the development of aneurysms were identified. On the basis of this study we recommend that patients with ectatic aortas have a repeat ultrasound 5 years after the initial study.