Primary superior mesenteric venous thrombosis is sometimes preceded by peripheral thrombophlebitis. Inherited antithrombin-III deficiency is a recently recognized autosomal dominant trait, which is characterized by thrombophlebitis and pulmonary embolism. This case report illustrates many features of both entities and strongly suggest a causal relationship. While long-term therapy has yet to be established, prophylactic therapy is recommended when asymptomatic individuals with known antithrombin-III deficiency are at increased risk of thrombosis. The efficacy of heparin alone has been unreliable, whereas Coumadin has been encouraging. Antithrombin-III concentrates are being developed and theoretically should be helpful. Patients with thrombophlebitis or pulmonary embolism should be suspected of having antithrombin-III deficiency. Such individuals also represent one mechanism to explain "primary" mesenteric venous thrombosis.