Aneurysms in the portal venous system are relatively rare. We report the case of an extrahepatic portal venous aneurysm, detected incidentally by ultrasonography. The patient, a 75-year-old woman, was initially observed over 18 months, during which time, the aneurysm grew from 36 mm × 32 mm to 51 mm × 37 mm in size, without symptoms. Hemodynamic analysis employing computational flow dynamics technique showed obvious turbulence in the aneurysm, and the wall shear stress (WSS) against that part of the aneurysmal wall was greater than in other sites. To prevent complications such as spontaneous rupture and portal vein thrombosis, the aneurysm was resected, with reconstruction of the portal trunk. While careful follow-up is sufficient for most portal venous aneurysms, its enlargement could indicate possible spontaneous rupture. The increased WSS against part of the aneurysmal wall most likely accounts for the aneurysm enlargement in this case.