Superior mesenteric arteriovenous fistula is rarely encountered. Unrecognized in early stage, it can cause hazardous sequelae of portal hypertension. Herein we report a rare case of portal obstruction associated with an aneurysmal dilatation of the superior mesenteric vein, which was caused by a superior mesenteric arteriovenous fistula. The patient was a 74-year-old woman admitted for continuous hemetemesis, presenting with a palpable abdominal mass. The computed tomogram revealed a 6.5 cm superior mesenteric venous aneurysm and thrombotic portal venous obstruction. On account of impending shock, an emergent portal venous thrombectomy and portocaval shunt was carried out. Superior mesenteric arterial angiography performed 4 months later confirmed the diagnosis. Being without symptoms, the patient was clinically followed and remained in stable condition for 3 and a half years. The clinical presentation of a mesenteric arteriovenous fistula varies, with occasional gastrointestinal tract hemorrhage. When complicated with portal obstruction, the condition can be disastrous. We believe that creation of a porto-systemic shunt with resection of the arterio-venous fistula is the proper treatment. This patient is now under close observation.