Membranous obstruction of the inferior vena cava is a rare disease. The etiology of the membrane is believed to be thrombotic or congenital. In three of 11 siblings from a single family, symptoms of membranous obstruction of the inferior vena cava developed during early adult life. All had signs of more long-standing disease, as judged by the presence of collaterals, cirrhosis and, in one case, hepatocellular carcinoma. On family screening no further cases of membranous obstruction of the inferior vena cava were found. There was also no evidence of inherited defects in the natural coagulation inhibitors (protein C, protein S and antithrombin III) and plasminogen deficiency. This familial occurrence of membranous obstruction of the inferior vena cava supports a congenital etiology, although a thrombotic etiology cannot be totally excluded.