Background: No drug reduces the growth rate and rupture risk in small abdominal aortic aneurysms (AAA). Observational studies have shown that patients with diabetes have lower prevalence of AAA, which does not fit with its impact on other vascular diseases. Instead, this effect could be related to diabetic medication. The Metformin Aneurysm Trial (MAT) will test whether metformin reduces the risk of AAA rupture-related mortality or AAA surgery in patients with small, asymptomatic AAA. Methods: MAT is an international, multicentre, prospective, parallel-group, randomised, double-blind, placebo-controlled trial. Patients were included if they had an infrarenal AAA ≥35mm, not planned for repair, not diabetic and not on metformin, with eGFR≥45. All patients undergo a single-blind run-in period prior to randomisation, where they receive metformin to assess whether they tolerate it. If tolerated, patients were randomised to either 1500mg metformin XR daily or an identical placebo. The follow-up period is 3.5 years, with primary outcomes being AAA rupture-related mortality or need for surgical repair. Secondary outcomes were AAA growth, major adverse cardiovascular events, and health related quality of life. 1954 patients are required for 90% power (p=0.05) to detect a greater than 25% reduction in risk of the primary outcome. Results: As of 01/07/2023, 17 sites in Australia and New Zealand are recruiting, randomising, and following patients up per the MAT protocol. Ten additional sites are being set-up; six in the UK are expected to begin recruitment this year. 366 patients have been recruited and screened, 304 entered the run-in period, 58 not tolerating metformin and hence excluded. Currently 247 patients have been randomised, with 175 patients reaching 12-month and 100 at 24-month follow-up. Conclusion: MAT could be important in guiding the treatment of small AAAs. Additional active sites across Australia and New Zealand would be of great benefit to the trials success.