Only scattered reports of portal vein and superior mesenteric vein aneurysms appear in the literature. Case reports of three patients with portal vein and superior mesenteric vein aneurysms diagnosed by computed tomography (CT) and gray-scale, color Doppler, and duplex Doppler sonography are presented. In one case, an isolated portal vein aneurysm was demonstrated. In the second case, an aneurysm of the portal vein and superior mesenteric vein resulting in biliary ductal dilatation was observed. In the third case, an isolated superior mesenteric vein aneurysm was found. None of the patients had a history or clinical evidence of underlying liver disease, pancreatitis, or other disease states that would predispose them to the development of aneurysms. The clinical presentations, possible etiologies, and imaging features of portal vein and superior mesenteric vein aneurysms are reviewed. The value of CT and sonography in the detection and characterization of these rare aneurysms is discussed.