This protocol tests whether a new anticoagulant, lepirudin, will be superior to heparin in preventing withdrawal occlusion of tunneled, open-ended venous access devices (VADs). At the Clinical Center approximately 25% of such VADs develop withdrawal occlusion requiring treatment with recombinant tissue plasminogen activator (rtPA). Because withdrawal occlusion is frequently caused by the accumulation of clot at the catheter tip, open-ended VADs are routinely flushed with heparinized saline. In theory lepirudin should be more effective in preventing accumulation of fibrin at the catheter tip because it can counteract the potent clotting enzyme thrombin that is bound in the fibrin, whereas heparin is less able to do this. In this protocol the assignment of the flush solutions is randomized. Because most withdrawal occlusion presents within a few weeks of catheter insertion, blinded flushes of heparin or lepirudin are given at least daily for the first 3 - 4 weeks of catheterization. Then all VADs begin unblinded routine flushes with heparin. The primary study endpoint is the number of patients in each group requiring rtPA because of withdrawal occlusion during the first 4 months after a VAD is inserted. To detect a 50% reduction in rtPA usage with a one-sided significance criterion of 0.05 and a power of 0.80, 147 patients will be required in each treatment arm, although the accrual goal is 170 patients per arm to allow for a small number of unevaluable subjects. Since most tunneled, open-ended VADs at the Clinical Center are used by patients on bone marrow transplant protocols of the NCI and NHLBI, these populations will be the primary sources of participants.