Severe hepatic venous outflow obstruction and its manifestations often are recorded under the label "Budd-Chiari syndrome." Unfortunately, this label is ambiguous; it does not clearly identify the site of the lesion (hepatic veins versus inferior vena cava), its morphologic features (thrombotic versus nonthrombotic), or its cause. In the literature, implied or expressed definitions vary. Use of a standardized topographic and pathogenetic classification of hepatic venous outflow obstruction would enable investigators to group patients with comparable conditions, as required for therapeutic trials, prognostic evaluations, and studies of pathogenetic pathways. Review of our own cases revealed that hepatic venous outflow obstruction involving large hepatic veins is usually thrombotic and that isolated obstruction of the inferior vena cava or of small hepatic veins is usually nonthrombotic. Application of such a classification seems feasible and may yield useful results.