This report documents a unique case of Budd-Chiari syndrome associated with antithrombin III deficiency and massive thrombus in the superior vena cava and right atrium. This aberration of the coagulation mechanism is proposed as an etiologic factor in the pathogenesis of hepatic venous obstruction whenever the cause is obscure. Antithrombin III was restored to a level adequate to permit thrombus extraction with cardiopulmonary bypass and relief of portal hypertension with a mesoatrial shunt. The protocol for reversing this hypercoagulable state with fresh frozen plasma and warfarin (Coumadin) is discussed, and technical innovations employed in the management of this complex problem are described.