Forty-one central catheters placed in 37 patients were subjected to a twofold phlebographic study - classic angiography of the main veins, and injection of a radio-opaque fluid while withdrawing the catheter under fluoroscopy. Although data from the literature shows a high incidence of venous thrombosis due to central catheters, we only found 3 positive results in our series of 37 patients (8%). A fibrin sheath develops along the entire length of the catheter, and, in cases of thrombosis, the point at which the catheter penetrates the central venous system is always involved.