 A total of 30 patients will be enrolled in this study. They will be divided into 3 groups according to their age range 6 months-age to 5, 5 to 12 years and 12 to 18 years. The patient must have a deep vein thrombosis clot in an artery or vein that has been seen on an imaging test. After receiving the study drug, the subjects will then have blood tests at specific timepoints to measure how the drug is effecting the body and how the drug is acting once it is in the body. Based upon the levels of the blood tests, the dose of the study drug may be adjusted or stopped. The subject will continue on the drug until the clot is dissolved, the subject is prescribed a different anticoagulant or the physician or parents decide to remove the child off of the study. The drug will also be discontinued if any excessive bleeding or severe side effects related to the drug are observed. A follow up imaging study will be performed on all patients 48-72 hours after the drug was started. If the clot is still present at this time, another imaging study will be done at 30 days after the drug was given. The subjects will continue to have lab tests twice weekly for about 3 weeks to monitor for the effects of the drug. After completion of the study, the patient will undergo a study exit physical exam between 20-40 days after the drug was stopped. 