Various medical therapies have been proposed to prevent abdominal aortic aneurysm expansion. However, there have been very few randomized clinical trials to support use of any of these treatments. Several animal studies and observational reports suggest that ARBs can be useful in reducing abdominal aortic aneurysm (AAA) growth. However, so far, ARBs have not been evaluated in a randomized clinical trial. Therefore, the purpose of the study is to evaluate the effect of valsartan, an ARB, on annual growth rate in comparison with atenolol, a beta-blocker. Our hypothesis is that valsartan is superior to atenolol in the suppression of the aneurysm growth at 12 months. The BASE trial is designed as a investigator-initiated, multi-center, randomized controlled open-label trial. Patients with small AAA (aorta diameter <5cm) will be randomized 1:1 either to valsartan or to atenolol group. Randomization will be stratified by the AAA size (max. diameter >4 cm or ≤4 cm). Patients will receive either valsartan (daily dose 80 mg or more) or atenolol (daily dose 50 mg or more) for 12 months. A CT scan will measure the maximal diameter of AAA at baseline and 12 months. The annual growth of AAA will be compared between the valsartan and the atenolol group.

Inclusion Criteria:

• Age ≥20years
• Abdominal aortic aneurysm with maximal diameter less than 5cm
• Hypertension
• Patient with signed informed consent

Exclusion Criteria:

• Saccular type aneurysm, inflammatory aneurysm, or infected aneurysm
• Aortic dissection
• Planned surgery or endovascular therapy for abdominal aortic aneurysm within 1 year
• Previous aorta surgery or endovascular therapy
• Contraindications to Beta-blocker or ARB (allergic reactions, asthma, severe bradycardia, angioedema, hyperkalemia)
• Allergic reaction to contrast dye
• Known genetic aorta disease or autoimmune or connective tissue disease: Marfan syndrome, Shprintzen-Goldberg syndrome, Loeys-Dietz syndrome, Takayasu's arteritis, Behcet's disease
• Pregnancy
• Life expectation <1 year
• Renal failure (serum Cr >2.0 mg/dL)
• Liver disease (ALT or AST > 3 x upper limit) or liver cirrhosis (Child B or C)
• Malignancy requiring surgery or chemotherapy within 1 year after enrollment
• Status post transplantation or chronic inflammatory disease requiring immune suppressive drugs over 4 weeks