Antithrombin III is a potent coagulant inhibitor in plasma. Congenital deficiency of antithrombin III may predispose to thrombotic events and may complicate surgical management. We describe a patient with congenital antithrombin III deficiency who developed superior mesenteric vein thrombosis after the cessation of warfarin therapy which resulted in venous gangrene of the small intestine. Initial treatment of this deficiency with fresh frozen plasma and subsequent long-term management with warfarin therapy has been effective in avoiding further thrombotic events.