Prompt diagnosis of aortoenteric fistulas is imperative for patient survival. The clinical signs of aortoenteric fistula include hematemesis, melena, sepsis, and abdominal pain, but the condition also may be clinically occult. Because clinical signs may not be present or may not be sufficiently specific, imaging is most often necessary to achieve an accurate diagnosis. Although no single imaging modality demonstrates the condition with sufficient sensitivity and specificity, computed tomography (CT), owing to its widespread availability and high efficiency, has become the imaging modality of choice for evaluations in the emergency setting. CT has widely variable sensitivity (40%-90%) and specificity (33%-100%) for the diagnosis of aortoenteric fistulas. To use this modality effectively for the initial diagnostic examination, radiologists must be familiar with the spectrum of CT appearances. Mimics of aortoenteric fistulas include retroperitoneal fibrosis, infected aortic aneurysm, infectious aortitis, and perigraft infection without fistulization. Differentiation is aided by the observation of ectopic gas, loss of the normal fat plane, extravasation of aortic contrast material into the enteric lumen, or leakage of enteric contrast material into the paraprosthetic space; these features are highly suggestive of aortoenteric fistula in a patient with bleeding in the gastrointestinal tract. © RSNA, 2009.