The systematic search for a deficiency in antithrombin III must be considered in case of: venous thrombosis in a young patient, recurrent venous thrombosis especially if these occurred under Heparin, venous mesenteric infarction since this type of thrombosis is rare and seems relatively frequent in case of congenital deficiency in antithrombin III, familial past history of venous thrombosis in a woman desiring to undergo estrogen-progesterone therapy. The most often used techniques are: study of antithrombin III activity by amidolytic method and titration by immunodiffusion. Anti-vitamin K treatment is the only effective therapy proposed to patients suffering from a hereditary deficiency in antithrombin III.