It is generally believed that membranous obstruction of the inferior vena cava in the Budd-Chiari syndrome is caused by congenital malformation. However, it does not explain the late onset of the disease. In the current study, hepatic portion of the inferior vena cava and hepatic veins were studied in 17 autopsy cases of the Budd-Chiari syndrome, 16 of which had no demonstrable cause (idiopathic). A sufficient amount of vena cava tissue was available for evaluation in 15 cases. Nine had membranous obstruction, with thickness varying from 3 to 8 mm. Thrombus formation was recognized in 7 of 9 cases. Occlusion of hepatic vein orifices of varying degree was present in 8 cases. In these occluding lesions, the basic structure of the venous wall was maintained. The intima was transformed into a fibrous laminar structure, and organized thrombi of varying ages were recognized; they were a mixture of fresh thrombi, organized thrombi, fibrous tissues, recanalizations, and calcifications. It is concluded that in these cases of the Budd-Chiari syndrome, occluding and stenosing lesions in the inferior vena cava and hepatic veins were thrombosis and its sequelae. There was no indication of congenital malformation.