Using Doppler ultrasound, we prospectively evaluated the relationship between central venous blood flow and the development of central venous thrombosis in ten patients with a long term central line. The presence of turbulent blood flow around the catheter was followed by the development of central venous thrombosis in two patients but neither had subsequent clinical sequelae. More surprisingly, however, normal blood flow was demonstrated in eight patients, two of whom subsequently developed intravascular thrombi.