A bdominal aortic aneurysm (AAA), an inflammatory disease, involves leukocyte recruitment, immune responses, inflammatory cytokine production, vascular remodeling, neovascularization, and vascular cell apoptosis, all of which contribute to aortic dilatation. This study demonstrates that mast cells, key participants in human allergic immunity, participate in AAA pathogenesis in mice. Mast cells were found to accumulate in murine AAA lesions. Mast cell–deficient Kit^W-sh/Kit^W-sh mice failed to develop AAA elicited by elastase perfusion or periaortic chemical injury. Kit^W-sh/Kit^W-sh mice had reduced aortic expansion and internal elastic lamina degradation; decreased numbers of macrophages, CD3⁺ T lymphocytes, SMCs, apoptotic cells, and CD31⁺ microvessels; and decreased levels of aortic tissue IL-6 and IFN-γ. Activation of mast cells in WT mice via C48/80 injection resulted in enhanced AAA growth while mast cell stabilization with disodium cromoglycate diminished AAA formation. Mechanistic studies demonstrated that mast cells participated in angiogenesis, aortic SMC apoptosis, and matrix-degrading protease expression. Reconstitution of Kit^W-sh/Kit^W-sh mice with bone marrow–derived mast cells from WT or TNF-α^–/– mice, but not from IL-6^–/– or IFN-γ^–/– mice, caused susceptibility to AAA formation to be regained. These results demonstrate that mast cells participate in AAA pathogenesis in mice by releasing proinflammatory cytokines IL-6 and IFN-γ, which may induce aortic SMC apoptosis, matrix-degrading protease expression, and vascular wall remodeling, important hallmarks of arterial aneurysms.