In Austria, about 2–6% of all men above 65 years are affected by an abdominal aortic aneurysm (AAA) which is a degenerative dilatation of the aorta beyond 1.5 times the normal size. AAAs are considered to be responsible for 1.3% of all deaths in men aged between 65-85 years. The expansion of the abdominal aorta may be tolerated until a size of 5.0-5.5 cm, when the risk for rupture increases substantially and patients require surgical repair. An effective medication to slow the progressive dilatation of the aorta and avoid high-risk surgery at advanced age has not been found to date. Previous studies suggested that diabetic patients are less likely to develop AAA and once diagnosed with AAA have slower progression. However, in a recently published study with diabetic AAA patients and different classes of medication, only the intake of metformin was protective against the annual AAA enlargement. Metformin seems to be reducing AAA growth, as further evidenced in a mouse model. We thus hypothesize that metformin might also protect against disease progression in AAA patients who do not suffer from diabetes and propose to conduct a clinical trial with 170 non-diabetic patients with an infrarenal abdominal aortic aneurysm of 4-5 cm. After a 2-week test phase to exclude patients with gastrointestinal side effects to metformin, the remaining patients will be divided into two therapy arms (in a doctor- and patient-blinded fashion) and treated for 12 months with placebo or metformin at 2000 mg per day. Aneurysm growth will be monitored by computed tomography every 6 months. If successful, metformin would constitute the first effective, low-cost medication to slow disease progression and thereby delay or avoid high-risk and high-cost surgical aneurysm repair.