We describe a 43-year-old male patient with congenital antithrombin III deficiency requiring haemodialysis due to extension of venous thrombus from recurrent deep vein thrombosis. During dialysis with adequate heparinization, the patient often revealed clot formation in the extracorporeal circuit resulting in unexpected discontinuation of dialysis. When either a combination of antithrombin III concentrate plus heparin or the newly developed synthetic antithrombin preparation, MD805, was infused during dialysis, he could be uneventfully dialysed with either of the two regimens. The functional antithrombin III activity with MD805 increased to the same level as that obtained with antithrombin III concentrate, and it was possible to achieve an antithrombotic effect, as measured from the APTT and thrombin-antithrombin III complex with MD805 during and after dialysis. We thus found that MD805 could be used as an anticoagulant drug when an AT III-deficient patient required anticoagulation in the extracorporeal circulation.