BACKGROUND AND AIMS: Abdominal aortic aneurysm (AAA) is a life-threatening vascular disease when aortic rupture occurs, especially for elders. There is an urgent need to understand the mechanisms of AAA formation and development at molecular level. Our previous study showed that disintegrin and metalloprotease 10 (ADAM10) played an important role in abdominal aortic aneurysm formation. In this study, we investigated the effects of another ADAM protein (ADMA9) in AAA formation. METHOD AND RESULTS: Using AngII treated human aortic smooth muscle cells (HASMCs) and human aortic endothelial cells (hAoECs) as in vitro AAA model and murine AAA model, ADAM9 was overexpressed suggesting that ADAM9 may play important roles in AAA formation. Further investigation showed that ADAM9 induced inflammation leading to increased macrophage infiltration. ADAM9 was also found to induce cell apoptosis. AKT/NF-κB pathway was activated in murine AAA. Bioinformatic analysis showed that the 3' UTR of ADMA9 was a potential target of miR-126. We investigated the potential of using miR-126 to modulate ADAM9 expression. The expression level of miR-126 was decreased and inversely correlated with the expression of ADAM9 in the in vitro AAA model. Further investigation showed that miR-126 negatively regulated gene expression of ADAM9 and suppressed the production of inflammatory cytokines. miR-126 was also found to improve cell survival and significantly reduce AAA formation in murine AAA. CONCLUSIONS: Our data revealed a link between ADAM9 and AAA formation, providing an approach to control AAA development using miR-126, possibly through modulation of the expression level of ADAM9.