Before operating on a case of iliac or ilio-caval phlebitis with or without pulmonary embolism, three questions need to be asked about the condition of the inferior vena cava: is it involved? is there a local cause? where is the superior pole of the thrombus in relation to the renal veins? The study of 10 cases of recent phlebitis benefitting from phlebography and C.T. scan reveals the value of this investigation. In 7 cases the condition of the lumen of the inferior vena cava was defined and in the 3 cases in which it was thrombosed, the position of the thrombus in relation to the renal veins was visualised. In 2 cases, the inferior vena cava was laminated by an extrinsic compression which was defined by the C.T. In only one case was the vena cava not seen, due to the presence of a large collateral channel. The indications for C.T. scan should remain selective: cases where direct ilio-cavography is impossible and where descending cavography is undesirable or impossible.