<qa>  What is the most common cause of deep vein thrombosis?Deep vein thrombosis of the central veins of the upper extremity and neck is most commonly seen at the Clinical Center as a consequence of use of central venous access devices VADs required for intravenous medication treatment programs at the NIH. In order to relieve discomfort associated with this condition or restore patency of the central veins, we successfully developed a treatment program employing intraclot pulse-sprayed injection of recombinant tissue plasminogen activator rtPA and anticoagulation with heparin plus warfarin Protocol 95-CC-0053. Recent laboratory kinetic studies of thrombolysis with rtPA suggest that more rapid thrombolysis can be achieved with rtPA doses that are more than 10 times lower than the doses used in our earlier protocol. The purpose of this protocol is to evaluate whether these lower doses translate into more effective thrombolysis in the clinical setting, so that we may modify our thrombolysis treatment to make it more effective, potentially safer, and less costly than our previous protocol. This proposed protocol still utilizes pulse spray injection of rtPA directly into the thrombus as before, but evaluates the efficacy and safety of using a total of 4 mg or less of rtPA each day for thrombolytic therapy. As this lower dose of rtPA has been found to be safe in catheter clearance applications, hospitalization will not be required as part of the protocol. Patients who elect to have this procedure performed as outpatients will also receive anticoagulation with standard regimens using subcutaneous low molecular weight heparin LMWH alone, or with gradual conversion to oral warfarin. 