Inflammatory abdominal aortic aneurysm (AAA) accounts for 5% to 10% of all cases of AAA and differs from typical atherosclerotic AAA in many important ways. Although both inflammatory and atherosclerotic AAA most commonly affect the infrarenal portion of the abdominal aorta, patients with the inflammatory variant are younger and usually symptomatic, chiefly from back or abdominal pain. Unlike patients with atherosclerotic AAA, most with the inflammatory variant have an elevated erythrocyte sedimentation rate or abnormalities of other serum inflammatory markers. Computed tomography and magnetic resonance imaging are both sensitive for demonstrating the cuff of soft tissue inflammation surrounding the aneurysm that is characteristic of inflammatory AAA. In contrast to atherosclerotic AAA, the inflammatory variant is characterized pathologically by marked thickening of the aneurysm wall, fibrosis of the adjacent retroperitoneum, and rigid adherence of the adjacent structures to the anterior aneurysm wall. An extraordinary expansion of the adventitia due to inflammation also distinguishes inflammatory from atherosclerotic AAA. Although the pathogenesis of inflammatory AAA appears to involve an immune response localized to the vessel wall, the etiology of the inflammatory reaction is unknown. Inflammatory AAA is almost never associated with inflammation of other arteries. Male sex and smoking, the main risk factors for atherosclerotic AAA, are even stronger risk factors for the inflammatory variant. Smoking cessation is the first step of medical therapy. Corticosteroids or immunosuppressive therapies may also have roles. Although inflammatory AAA appears less likely to rupture than atherosclerotic AAA, surgical intervention appears prudent once the diameter of the aneurysm exceeds 5.5 cm. Knowing the features of inflammatory AAA should allow physicians to distinguish it from atherosclerotic AAA or from systemic vasculitis and to treat it with the appropriate combination of medical and surgical therapies.