In a pilot study of continuous infusion 5-fluorouracil and intermittent bolus doxorubicin and cyclophosphamide in women with breast cancer, four of 24 patients developed symptomatic superior vena cava or innominate vein thrombosis associated with the Hickman line, despite prophylactic treatment with very low dose warfarin (1-3 mg/day). In all four patients, local thrombolysis with streptokinase was successful and chemotherapy was continued through the Hickman line under anticoagulant cover, maintaining an international normalized ratio of 2.0-3.0. No patient developed recurrent thrombosis. Prophylactic anticoagulation should be considered in patients receiving continuous infusion chemotherapy through Hickman lines, as they are at risk of proximal vein thrombosis. A randomized study is needed to address the question of the optimum anticoagulant regimen to prevent such thromboses.