Purpose: The expansion rates and outcomes of lilac artery aneurysms (IAAs) were determined. Methods: A retrospective chart review was conducted to identify patients in whom IAA had been diagnosed between June 1990 and March 1999 in a vascular surgery service at a large university-affiliated Veterans Affairs medical center. The patients were veterans, 187 men and two women, in whom the diagnosis of an IAA was made, as defined by the Ad Hoc Committee on Reporting Standards of The Society for Vascular Surgery and International Society for Cardiovascular Surgery, North American Chapter (IAA ≥ 1.5 cm). Expansion rates relative to the size of IAAs and clinical outcomes were noted for all patients. Results: One hundred eighty-nine patients (mean age, 72.3 ± 0.5 years) with 323 IAAs (mean size, 2.34 ± 0.7 cm) were found. The mean follow-up (96% of patients with B mode ultrasound scanning) period was 31.4 months, with each patient undergoing a mean of 4.2 studies. The 4-year life-table survival rate was 78.2%, with no patient deaths related to their IAAs. Symptoms were noted in six of 189 patients (3.1%; two ruptures, four chronic pain), who all had IAAs larger than 4 cm. IAAs were repaired in 34 of 189 patients (18%), in 25 of the 34 patients because of their associated abdominal aortic aneurysms and in nine of 34 patients because of their IAAs alone. All nine patients requiring operative treatment of indications related to the IAA had an IAA larger than 4 cm. Expansion rates were slow for IAAs smaller than 3 cm (0.11 ± 0.02 cm/year) and significantly greater (P < .003) for IAAs 3 to 5 cm (0.26 ± 0.1 cm/year). The correlation between B mode ultrasound scanning and computed tomography scanning was excellent. The size of the IAAs was underestimated by 0.03 ± 0.06 cm by means of B mode ultrasound scanning. Conclusion: The IAAs followed up by this contemporary Veterans Affairs vascular surgery service were small, rarely caused symptoms or rupture, and expanded at a slow rate. IAA smaller than 3 cm could be followed up safely on an annual basis with B mode ultrasound scanning. IAAs that are 3 cm or larger and smaller than 3.5 cm should be carefully followed with B mode ultrasound scanning at 6-month intervals, whereas elective repair should be considered for IAAs 3.5 cm or larger in good-risk patients. Based on this report and currently available evidence and recommendations, asymptomatic IAAs that are 4 cm or larger and all other symptomatic IAAs should be considered for operative repair. Also, the reported high rupture rate of IAAs that are 5 cm or larger mandates prompt operative repair.