We report the case of a 67-year-old man presenting with abdominal pain due to acute thrombosis of an aneurysm of the superior mesenteric vein. It was treated conservatively by a combination of anticoagulation and monitoring by serial imaging after multidisciplinary discussion. With more than one year of follow-up, no aneurysm growth was observed. Mesenteric vein aneurysm is a rare vascular dilatation, which was first described in 1982 and since then 17 cases have been reported. There are congenital and acquired hypotheses regarding their etiology. If asymptomatic, conservative management is recommended. Monitoring is important because of potential complications (thrombosis, portal hypertension, adjacent organs compression and rupture of aneurysm), which may mandate surgical management.