Thrombosis is the most frequent benign etiology of superior vena cava syndrome among cancer patients who have a long-term central venous catheter. In this paper, six cases of thrombotic superior vena cava syndrome are discussed. There were four women and two men. One patient was treated with streptokinase and five with urokinase. The mean age was 46 years (range 22- 69), and the mean time for thrombosis development after catheter insertion was 125 days (range: 53-211 days). The mean time for resolution of thrombosis was 7 days (range 2-11) in five patients. One patient had no response to fibrinolysis.