We report herein the case of a 20-year-old woman in whom an enormous aneurysmal portahepatic venous fistula was found. The portal aneurysm developed in the anterior inferior segment of the right hepatic lobe, and communicated with the right portal vein and the right hepatic vein. Due to the enormous size of the shunt, and because the patient's venous blood ammonia level increased by about twofold after eating, a right lobectomy of the liver was performed. A review of the available literature and a discussion of the most appropriate treatment follows the presentation of this case.