The causes, pathology, and clinical features of thrombosis within the portal venous system are discussed. When thrombosis starts in the periphery of the mesentery the extent of infarction in the bowel may be limited and treatment by resection of the thrombosed mesentery and adjacent gut may be successful. When the thrombosis is proximal thrombectomy is essential. A case is described in which proximal mesenteric venous thrombosis occurred in assoication with volvulus of a common embryonic midgut mesentery. Laparotomy was performed and a thrombus 25 cm long extending into the portal vein successfully removed by catheter thrombectomy.