Background: Flow dynamics are a significant factor in the development and possible rupture of the aneurysms. Therefore, it is close to reason to present detailed data on the realistic 3D surface and internal geometry of a primary aortoenteric fistula, i.e. of its fistulous tract. Methods: A primary aortoenteric fistula in a patient with an abdominal aorta aneurysm underwent quantitative analysis by means of multimodal image processing, based on multi-detector computed tomographic angiography dataset. The medical computer-aided design presented a 3D model with the fistula as a branch of the abdominal aorta. A full array of centerline calculations was performed on this fistula, three levels of the aneurysm and on the superior mesenteric artery. Results: The hydraulic properties of the fistulous tract were comparable to the superior mesenteric artery and even to the abdominal aorta aneurysm. Conclusion: 2D and 3D geometry measurements provide insight into the flow dynamics of the primary aortoenteric fistula, relevant for surgical treatment.